A Bioenergetics Approach For Determining The Effects Of Increased Striped Bass Population On It’s Prey and Health In The Chesapeake Bay
Maryland Department of Natural Resources
Tawes Office Building
Annapolis, MD 21401
Anthony S. Overton (Graduate Research Assistant)
Eric B. May (Project Coordinator)
Jennifer Griffin (Graduate Research Assistant)
F. Joseph Margraf (Principal Investigator)
Maryland Cooperative Fish and Wildlife Research Unit
University of Maryland Eastern Shore
1120 Trigg Hall
Princess Anne, MD 21853
This is a Cooperative Program of the U.S. Geological Survey
Maryland Department of Natural Resources
University of Maryland Eastern Shore
U.S. Fish and Wildlife Service
Wildlife Management Institute
The Chesapeake Bay Ecological Foundation, Inc. assisted in the collection of striped bass examined for this project and is making this report available on our web site at www.chesbay.org a final analysis of this study will be completed in 2001.
The purpose of this study is to determine the effects of the increased striped bass population on its prey, develop an historical profile of the feeding habits and predatory demand of striped bass from 1955 to 1970, and identify the influence of mycobateria on the over all health of striped bass population in the Chesapeake Bay.Since the removal of the moratorium in 1990, the striped bass population has been increasing in the Bay.This increase in striped bass biomass may have adverse affects on the prey fish community. Changes in the relative abundance and composition of the prey fish community may also influence the health of striped bass by rendering striped bass more susceptible to bacterial infections. Understanding the historical feeding habits of striped bass may provide key information to any changes in diet which may be directly related to prey abundances. Several species of bacteria have been isolated from striped bass in the Chesapeake Bay. Mycobacteria sp. is of particular interest to scientist because of its association with large nodules (granulomas) or ulcers throughout many of their tissues. The resulting disease may be characterized by emaciation, inflammation of the skin, exophthalmia (Pop-eye), ascites (Dropsy), open lesions, and ulceration. We examined the diet of striped bass to determine composition of the diet and also provide information about the food habits of striped bass from 1955 through 1970. These data were used with bioenergetics modeling to estimate predatory demand of striped bass. We also determine the extent of Mycobacteria infection in striped bass and provide evidence of decreased overall condition of fish affected by this bacteria.
Striped bass fed on a wide variety of prey items. Sand shrimp were the most numerous (59%) of prey items found in the diet followed by Bay anchovy (28%) ,Blue crab (5%),and Atlantic Menhaden (3%).The highest SFI index was during the July-August sample period(7.42)and the lowest during the May- June(1.00) period. Menhaden contributed the largest portion to the biomass (48%) followed by gizzard shad (15%) bay anchovy (11%), and blue crab (7%). The bay anchovy had the highest percent frequency of occurrence (34%) and the highest IRI value of (45%). Individual consumption increased with age. Bay anchovy contributed between 47-65% to the annual consumption. Daily consumption of menhaden was significantly less than consumption in 1993 but blue crab consumption was significantly greater. During the period of 1955 through 1959 the most numerous prey items identified were bay anchovy (40%) followed by Atlantic menhaden (40%). Almost 50% of the fish showed some sign of external sores. Granulomas appeared in at least one organ in 53% of the fish in our samples, regardless of the presence or absence of external sore. The Gran scores from the spleen and heady kidney were significantly higher than either the heart and liver and appeared to be most closely associated with the presence of external sores. The condition factor was significantly higher for stripped bass without sores (0.96) than those with sores (0.82). Condition factors involving all fish clearly shows a trend, with NGNS at 0.99,GNS at 0.92 and GS at 0.81. Slopes of the regression loge weight (g) and loge length (mm) for striped bass with sores was significantly higher than those fish without sores. However the weight at length were more variable (lower r2) in striped bass with sores than those fish without sores.
General overview – The Chesapeake Bay serves as a nursery area for many resident and migratory species. This dynamic ecosystem supports multiple fisheries including a striped bass fishery, which is both commercially and recreationally important in the Chesapeake Bay. In the 1970’s and 1980’s striped bass stocks in the Chesapeake Bay declined. Commercial landings peaked to 6,300 mtin 1973 and declined to 772 mtin 1983 (AFMFC 1983). In January of 1985, the Atlantic States Marine Fisheries Commission (AFMFC) placed a moratorium on the catch, sale or possession of striped bass, on the Atlantic coast in an effort to restore the population. Toxic contaminants, starvation and predation of larvae, overfishing, and competition for food and space were believed to be the cause of striped bass population declines. Following evidence of successful reproduction with the juvenile index exceeding a three year running average of 8, the AFMFC (1995) declared the Atlantic coast striped bass fully recovered in 1995. Management efforts have continued and the striped bass population is increasing which is evident from commercial catch data. (Figure 1).
The striped bass is one of many piscivorus estuarine species (Manooch 1973;Chao and Musick 1977) and plays an important ecological role as a predator. As such, striped bass are either affected by the abundance of prey (bottom-up control) or can themselves serve to regulate prey abundance (top-down control) (Carpenter et al. 1987). Stripe bass are opportunistic feeders primarily feeding on soft-rayed fishes (Hollis 1952). The dominant prey consumed varies with habitat depending on prey abundance, environmental factors, habitat, and prey availability (Stevens 1958;Manooch 1973). Changes in striped bass diets in the Chesapeake Bay may reflect to changes in its prey populations (Hollis 1952). Bay anchovy and Atlantic menhaden were the dominant prey for striped bass in the summer and fall, and juvenile spot and Atlantic croaker were the dominant prey in winter (Hollis 1952). However in later studies menhaden, bay anchovy and spot were the primary prey items throughout the year (Hartman 1993). These apparent shifts in food habits of striped bass may represent changes in prey abundance.
Recent studies have focused on the predatory effect of stripped bass on its prey items in the Chesapeake Bay. Such prey species as blue crab and Atlantic menhaden, support an important commercial fishery in Maryland. Hartman (1993) found that stripped bass collected near the Patuxent River consumed Atlantic menhaden which increased with age with the annual incidence in striped bass diet rising from 33% at age 1 to 66% at age 6.
Bioenergetics – Bioenergetics models are used by fisheries biologists in a variety of ways to address many current ecological problems, but usually used to estimate food consumption based on growth, diet, and temperature data. (Tyler and Calow 1985; Beauchamp et al.1989; Rice and Cochran 1984;Minton and Mclean 1982). Bioenergetic models have also been used to predict predation rates of fish (Kitchell 1983;Stewart and Ibarra 1991; Hartman and Margraf 1992; Hartman1993) and have become increasingly useful in understanding predator prey dynamics.
The basic use of boienergetics models is to predict consumption (daily) rates based on initial and final mass of fish over time. It is essentially a balanced energy equation on which consumption is equated to the sum of total metabolic activities.
Bioenergetic model requirements vary depending on the model application. The model requires the input of environmental temperature of the predator, energy densities (joules or calories) of predators and their prey. Energy densities often differ over time and may vary seasonally with life stage. A measure of growth to estimate consumption or a measure of consumption to estimate growth is also needed as input data for the model.
There is an increasing emphasis on multi-species management as an approach to maintain ecological balance, while at the same time providing recreational and commercial fishing opportunities. This forces management agencies to utilize different approaches to assess the effects of any one species on food web dynamics, which may be particularly difficult in large estuarine systems such as the Chesapeake Bay. This is made even more complicated when the effects of the increased striped bass population in the Chesapeake Bay have to be considered. Increased numbers of striped bass places a greater demand on their primary and alternate prey. Because of this, there is concern about the potential effect of the increased striped bass population on blue crabs, out-migrating juvenile alosids and other important prey by striped bass in the Chesapeake Bay Estuary.
Health Considerations – In addition to concerns related to the potential predation on blue crabs and Atlantic menhaden, have been the appearance of underweight striped bass and striped bass exhibiting external sores in the Chesapeake Bay. The appearance of sores on fish is not considered uncommon ( May and Sindermann, 1999),however since 1994 the occurrence of these sores on striped bass has been increasing ( Unpublished Data, May 1998).When first reported in the Wicomico River ( Western Tributary of the Potomac) of Maryland, isolates of Edwardsiella tarda were obtained from fish exhibiting signs of the condition ( Baya, et al., 1997). This pathogen had never been linked to diseases in wild fish and was the first report to suggest that E. tarda could affect wild populations. By later summer through fall of 1997, 10 % of striped bass sampled in the Chesapeake Bay had observed lesions and nearly 13% through October 1998 (MDNR, 1998; Waller et al. 1997). Prior to 1997, additional isolates were obtained from striped bass showing clinical signs of infection, E. tarda could not be re-isolated, however a variety of other gram negative enterics were isolated including Aeromonas, Pseudomonas, and Vibro sp. (MDNR, 1997; 1998).Including and since 1997 the most of bacterial isolates identified from symptomatic striped bass have been Mycobacterium sp. It is believed that Mycobacteria were responsible for many of the sores identified in the striped bass taken from the Potomac in 1997 and 1998 (Vogelbiem et al., 1999).
Since 1996, fisherman from the Chesapeake Bay expressed concern regarding the disappearance of many of the species utilized by striped bass as forage, particularly menhaden. These observations have been supported by Maryland Department of Natural Resource reports (Uphoff 1998) which suggest that populations of Atlantic menhaden, silverside, spot, and other forage species have been declining based on decreased harvest and reduced appearance in collections in MDNR annual juvenile seine surveys. Uphoff (1998) found a correlation between striped bass declining prey abundance and the declining health of striped bass in the Chesapeake Bay.He suggested that the decrease in the strength of the length weight relationship and increased occurrence of external lesions is because of the declining menhaden population in the bay and an increased occurrence of unhealthy appearing striped bass.
Basis of Project Goals – If these observations are valid, then if would mean that there has been a loss of or shift in the prey being utilized by striped bass in the Chesapeake Bay. Based on the potential influence of striped bass on their prey and the apparent relationship between underweight fish and skin sores, work was initiated to examine the current dietary habits of striped bass and any relationship with diseases that affect the skin.
Thus the intent of the bioenergetic component of this project was to determine if there has been a change in the dietary demands of striped bass since the early 1950’s when such data was first being collected.To do this a historical assessment of predatory demand is needed which then can be compared with more current estimates such as those by Hartman (1993) and studies being conducted under the auspices of this project. To date, no study had quantitatively assessed the role of striped bass predation on other important Chesapeake Bay species during the period prior to the moratorium. Without this benchmark it would be difficult to determine or even suggest any shift in prey consumption by striped bass or alteration in the bioenergetics as a consequence of increased striped bass abundance. Concurrently, to address the relationship between striped bass diets and the appearance of sores on the skin, gross and histological data were obtained from the same fish as well as fish taken from other sources.
This report then is a compilation of three separate, inter-related studies that addresses the historical food habits, the current food habits, and the potential changes of these food habits on the health of striped bass.
Relationship Between Skin Sores on Striped Bass and Condition – The objectives for this component were based on data from this study and a companion study, and were:
- Determine incidence of external sores throughout the Chesapeake Bay
- Relate the appearance of sores to type infectious agent affecting the fish
- For fish infected with Mycobateria, correlate degree of infection with condition
Here we present evidence that daily consumption of prey items particularly menhaden and blue crab has shifted over time.We also have evidence that striped bass in the Chesapeake Bay that have become infected with organisms capable of inducing a granulomatous response or exhibiting external sores have lowered mean condition factor and demonstrate a greater variance in weight ranges at a given length.
Striped Bass were collected from April 1998 to December 1999 from the Maryland portion of the Chesapeake Bay. Each sampling site was identified by latitude and longitude coordinates (Figure 2) and vertical CTD profiles were taken coincident with collections to provide data on temperature, D.O., and salinity occupied by the predators.Most sample collection was during daylight hours however some sampling extended into the night hours.Fish were collected by electroshocking (at freshwater sites), drift gill nets, trawls (Age 1), and hook and line.Samples were collected cooperatively with several other agencies including the Maryland Department of Natural Resources (MDNR), US Fish and Wildlife Services (USFWS), and Chesapeake Bay Ecological Foundation (CBEF). Many anglers from commercial, sport, and recreational fishing community also provided additional samples and also assistance in the field collections.
Diet data collected in this study were used to estimate consumption with bioenergetic model. Individual predatory consumption (supply) was calculated for both the observed (supply) and the potential (demand) from the bioenergetics model.We were interested in the changes in the consumption rates from 1993 to the present. Present individual consumption estimates were compared to estimates in 1993 to examine any changes. We concentrated on the consumption rates on menhaden, blue crab, and bay anchovy.
Classification of Granulomas – To determine the degree to which a given organ was affected of if granulomas were present, each section of an organ was examined, four fields randomly selected, and at 10x, all granulomas counted.A granuloma had to lie completely with in the field to be counted. If there was not enough tissue to be examined the sample was not included in the analysis. From these counts a granuloma (Gran) score for each organ (the average number of granulomous nodules in each field) was generated.The decision to classify a fish as having or not having granulomas rested on finding a granuloma in any of the four organs examined. If a granuloma was found, the fish was designated as having a granuloma i.e. granuloma no sore GNS or granuloma sore (GS). If no granulomas were found in any of the four organs, the fish was designated as not having a granuloma i.e. no granuloma no sore NGNS or no granuloma sore (NGS). These categories are presented in table 5.
Classification of Granulomas- To determine the degree to which a given organ was affected of if granulomas were present, each section of an organ was examined, four fields randomly selected, and at 10x, all granulomas counted.A granuloma had to lie completely with in the field to be counted. If there was not enough tissue to be examined the sample was not included in the analysis. From these counts a granuloma (Gran) score for each organ (the average number of granulomous nodules in each field) was generated.The decision to classify a fish as having or not having granulomas rested on finding a granuloma in any of the four organs examined. If a granuloma was found, the fish was designated as having a granuloma i.e. granuloma no sore GNS or granuloma sore (GS). If no granulomas were found in any of the four organs, the fish was designated as not having a granuloma i.e. no granuloma no sore NGNS or no granuloma sore (NGS). These categories are presented in table 5.
Calculation of Condition Factor- We calculated Fulton Condition factor to compare the condition of fish in all assigned categories using the equation below:
K=weight (g)/length (mm)3
Statistical Analysis- For statistical purposes two assumptions were made; (1) fish which did not exhibit granulomas in any of the organs or external sores were healthy fish, and (2) because spatial distribution was not considered, all fish were susceptible to bacterial infection to the same degree and when exposed infected at the same rate.
Nonparametric statistical tests were used for comparisons due to non-normal distributions of Gran scores from adult striped bass. (SAS Institute, Inc., Cary NC). Granuloma scores were ranked according to sample (organ) and analyzed by the Kruskall-Wallis test against the null hypothesis that granuloma (Gran) scores (with respect to organ) would not differ between fish with and without skin lesions. Tukey’s multiple comparison test was used after a significant detection of differences between medians. Significance was set at p<0.05 in all runs. We used an analysis of covariance and compared the slopes of the line for the momgeneity of slopes of the line for homogeneity of slopes of length and weight between with external and no external lesions.
We used an ANOVA to compare condition factor (K) (log transformed) between fish with or without external lesions and also between fish with or without granulomous. As with granulomous score, Tukey’s multiple comparison test was used to detect differences following significance.
Striped bass condition. Striped bass weight-at-length was used to determine whether differences existed between striped bass with or without external sores.
Current Status- To date we have weight and length data from 5988 adult striped bass taken from 1954 to 1970. This data was provided by MDNR. Most of the diet data are from fish collected during 1955 through 1959 and 1968 and 1969. However, the 1968 and 1969 data only records a full or empty status and therefore were not included in the diet analysis. A synopsis of data collected are given in Table 11. Mean, minimum and maximum total length (mm) and weight (g) for all fish collected in each year are given in Table12. Data on diet composition were identified for 905 striped bass ages 1 through 18.Of these, 514 had material in their stomachs. During this period the most numerous prey items identified were bay anchovy (40%) followed by Atlantic menhaden (40%).Specific diet data for 1955 through 1959 of stomach content are summarized in tables 13 through 17.
Relationship Between Skin Sores on Striped Bass and Condition
Categorical Assignment Based on Granulomas- Histological examination of the granulomas stained with both hematoxylin and eosin (HE) and Zeil-Nielsen (ZN) revealed that in the 35 samples where granulomas were designated to be induced by mycobacterial infections, all granulamas designated as microbacterial induced contained acid fast positive organisms.These organisms stained intensely with the stain and morphologically resembled Mycobacterium sp. (Frerichs 1993). The characteristics for designating a granuloma as being induced by mycobacteria were the aggregation the of epitheliod cells around a core of necrotic debres.
We had varied results in the categorical assignment of the fish in our samples. As expected all fish which showed signs of external sores did not have granuloma present in any of the organs.Almost 50% of the fish showed some sign of an external sore. Granulomas appeared in al least one organ in 53% of the fish in our samples regardless of the presence or absence of external sores. The fish with granulomas were further divided into 16% and 37% for the GNS and GS categories respectively (Table 18). Thirty –two percent of our samples were assigned to the NGNS and the remaining 15% into the NGS group.
When all fish are examined the Gran scores from the spleen and head kidney were significantly higher than either the heart and liver and appeared to be most closely associated with the presence of external sores.Gran scores for the GNS and GS categories were similar for the spleen and head kidney and both were significantly higher than the liver and heart (Figure 10). There was a trend suggesting that the spleens had higher gran scores than the head kidney on the average, there was no significant difference between the two organs. The GS gran score (2.11) was routinely higher that the GNS (0.59) treatment.When legal (greater than 457mm) and sublegal (less than or equal to 457) striped bass were separated (Figure 11) both showed similar patterns as above, however for legal fish there was a better association of the Gran score for the spleen and sores that the head kidney and sores.
Figure 11. Gran scores for each gross condition category for both legal and sublegal striped bass from Chesapeake Bay. Granulomas were not present in the fish from the categories NGNS and NGS so their gran score was 0.
Condition Factor- When all of the fish were combined (legal and sublegal) the condition factor was significantly higher for striped bass without sores (0.96) than those with sores (0.82) (Tables 19 and 20; Figure 12).Histologically, granulomas could not be identified in any of the organs of the NGS category of fish, and based on previous work (MDNR 1997) it must be assumed that the cause for the sores involves another infectious agent. For this reason comparisons of condition factor between NGNS and NGS categories will be limited to the combined sample (legal and sublegal) and no comparisons with the GNS or GS categories made.
Condition factors involving all fish clearly shows a trend, with NGNS at 0.99, GNS at 0.92 and GS at 0.81 (Table 19; Figure 13). The NGNS and GNS condition factors were not significantly different (p<0.05) from one another, but both were significantly different from GS category (p<0.05). When the sample was separated into legal and sublegal fish, this trend continued for the sublegal fish (Table 20; Figure14) but not for the legal fish.The legal fish had condition factors for the NGNS and GNS categories that were very similar, with the GS category condition factor lower than the other two, but not significantly so (Table 20; Figure 14).
Current Profile of Striped Bass Feeding Habits in the Chesapeake Bay
The current diets of striped bass were diverse and usually reflected the spatial and temporal abundance of prey items. Bay anchovy are distributed throughout the Chesapeake Bay and are accessible to most resident striped bass during the year. They contributed to 28% of the numerical and 11% of the total biomass. Bay anchovy were most numerous in the diet during the July-August sampling period. This also coincided with the highest period of the SFI. During this period we observed striped bass actively feeding on bay anchovy near the surface of the water. Most fish collected contained greater than 15 individual bay anchovy in their stomach. Some fish observed contained greater than 30 individual bay anchovy. This was the only time of the year when we collected striped bass feeding on the surface that contained such and abundance of bay anchovy in their stomachs. This may represent the time of the year when bay anchovy are most numerous and available to resident striped bass.
Although menhaden consisted of 3% of the numerical diet, it contributed to almost one half of the total diet biomass. Menhaden contributions to biomass was greatest during the November-December sampling period. We collected many of our larger striped bass (>635mm) during this time many of which had 1 to 2 menhaden greater than 250mm. Young menhaden migrate into the tributaries of the Chesapeake Bay in early May. These young-of-year (YOY) fish are available to striped bass however many fish in our samples in those areas did not contain YOY menhaden from May through mid September.In July and August menhaden contributed to only 0.49% of the biomass. It was during September and October when contributions of menhaden to biomass increased to 39%. MDNR juvenile index for Atlantic menhaden has been declining since 1985. The low abundance of YOY menhaden could be the possible reason for the absence in the diet from May through mid September.In our study mean daily consumption of menhaden for all age groups was consistently less than during 1993.In 1993 menhaden contributions to production ranged from 37-66% whereas during the current study contributions ranged from 12-27%. These differences may be because of the decline in recruitment of menhaden along the Atlantic coast. One may argue whether the decline in menhaden is because of the increase in the total consumption of the increasing striped bass population. Hence striped bass in the Chesapeake Bay have the ability to control their prey abundance. Striped bass are capable of controlling their prey abundance’s in large reservoirs (Moore, 1988). However it is unclear if striped bass can have the same influence on its prey items in the Chesapeake Bay. Although the stripped bass is a top piscviore, other predators rely on menhaden as a source of food (Hartman 1993). Thus the consumption habits of top predators must be considered in assessing the predation on menhaden. Declines in this once abundant prey species of striped bass may lead to shifts towards other nontraditional prey species or directly influence other commercially important species.
Striped bass have been known to consume blue crabs however their full potential influence on the fishery has not been assessed. Hartman 1993 estimated that two million age-2 striped bass annual consumption could equivalent to that harvested in Maryland in 1998.Daily consumption of blue crabs was significantly higher that estimated in 1993. Blue crab consumption by age 1 and 2 was less than one percent.Older fish depended more on blue crabs for production. This increase in consumption could possibly be contributed to the reduction of menhaden available in the Chesapeake Bay. If this is true striped bass could potentially reduce the blue crab population significantly. Spot and Atlantic croaker have traditionally been important to the diets of striped bass (Hollis 1952). MDNR juvenile index survey of those two species have remained constant for the past ten years.We would assume that any shifts in diet would be towards fish such as spot, Atlantic croaker, or other fish species. However these two species were present in very low numbers in our data.
We examined individual consumption and did not make estimates of consumption at the population level. Although estimates of individual consumption are important and may change from year to year, changes in predator populations should show the true influences of predation on prey species. The interest of fishery managers for the Chesapeake Bay is the effect of the increased striped bass population on fish species.
Striped bass diets, growth, predation, and consumption are influenced by changes in the density of striped bass, which may influence the relative abundance and composition of the prey fish community. The predatory affect of striped bass in the Chesapeake Bay Estuary in unclear. We hope to provide information leading to a better understanding of the role of striped bass in the Chesapeake Bay. These estimates will allow managers to more effectively deal with other resource user groups and provide reliable estimates of the results of management decisions for striped bass on other Bay resources.
Input data for bioenergetic models are important to the accuracy of the output of the model. We also used an age-length key to age our fish. True readings of age are needed to increase the accuracy of the consumption estimates on the cohort level. The use of an age-length key may have led to the minimal differences in consumption as a function of temperature observed in ages 2-4. The completion of the aging of striped bass will allow us to estimate the predation of striped bass to quantify the predatory demand more accurately at the individual, cohort, and population level.
Historical Profile of Striped Bass Feeding Habits in the Chesapeake Bay
During the period from 1955 through 1959 bay anchovy and Atlantic menhaden dominated the diet of striped bass. These two species contributed equally to the diet during this period.We have not been able to collect the desired amount of food habit data on striped bass from 1960-1970.However to complete this we will be contacting other sources for information specific to the diets of striped bass for inclusion into the bioenergetics model.
Relationship Between Skin Sores on Striped Bass and Condition
There is general agreement that systemic microbial infections in fish, particularly bacteria, lead to a combination of weight loss and the appearance of sores on the skin (Ferguson 1990; Roberts 1989; May and Sindermann 1999).What is not understood is why since 1994 there has been the repeated seasonal appearance of sores on striped bass.Early diagnostic work implicated E. tarda as the primary agent responsible for the condition (Baya, et al. 1997). A variety of bacteria have been isolated from striped bass exhibiting skin sores since 1997 (MDNR, 1998). The data from this study would suggest that at least 15% of the fish taken exhibited sores that were the result of non-mycobacterial infection.
From 1997 to the present the focus has been on Mycobacterium sp. because it has been this group of bacteria that have been consistently isolated from striped bass resident to the Chesapeake Bay.Mycobacterium sp. are acid fast bacteria belonging to the Mycobacteriaceae family.Characteristics of this group are that they are slow growing and extremely fastidious when cultured, making them difficult at best to isolate and identify (Frerichs 1993). Several members of the group have been found to infect fish, M. marinum, M. fortuitum, and M. chelonae (Frerichs 1993). More recent work by the Virginia Institute of Marine Science (VIMS) has shown that possibly 7 species could be involved, greatly complicating the picture (Vogelbiem et al., 1999).
Experimental evidence with mycobacterial infections has repeatedly shown that once infections are established, they are progressive leading to emaciation, appearance of sores, and ultimately death (Reimschuessel, 1997). The only experience with wild striped bass infected with Mycobacteria sp. was in the late 1980’s involving resident populations in the San Fransisco Harbor area (Sakanari et al. 1983), our understanding of how this genus of bacteria will affect striped bass in the wild is very poor. The wide spread distribution of the condition in striped bass since 1997 did, however, lead fishery managers to consider the possibility that the skin lesions were the consequence of infections which were brought on by the lack of available prey. This possibility was considered based on the number of emaciated appearing striped bass and the apparent relation between emaciation and the appearance of sores.
This phase of the study was designed to answer those questions which would be of importance to fishery managers in understanding what the potential influences of this condition are, and they are: (1) what is the degree to which the population is affected by either the infective agent, condition or both; (2) which came first, the infection or starvation what will be the final outcome of the infection or condition (will the fish survive) and (3) what will be the final outcome of the infection or condition (will the fish survive).
The incidence of sores from the fish taken during 1998 and 1999 was nearly 50%, of which 15% were represented by striped bass exhibiting sores for which the cause could not be attributed to mycobacteria. Fifty three percent of the striped bass taken in 1998 and 1999 had granulomas attributable to mycobacteria in one or more organs.This suggest an extremely high rate of infection. This rate is consistent with that seen in California and Oregon Waters in the early 1980’s with 25-68% infection in California, the highest in the San Francisco Bay area, and 46% in Oregon Water (Sakanari et. al. 1983).
Experimental work in the past has shown that otherwise healthy goldfish, when injected with Mycobacteria sp., will exhibit a progressively debilitating disease with death the ultimate outcome (Reimschuessel 1997). Similarly naturally infected populations of yellow perch, cod, farm-raised turbot and flounder exhibited the same progression with emaciation and death the outcome (Dalsgaard, et al. 1992; Daust, et al. 1989; Lama, et al. 1996; Vethaak and Jol 1996). The outbreak in San Francisco Harbor area was intensively studied but the final conclusions were not able to suggest that the affected fish died even though the authors felt that the condition would probably lead to death (Sakarnari et al. 1983).The results presented from this study to confirm the experimental work and the inferences of Sakanari et. al. (1983) that death is the outcome after mycobaterial infections.The condition becomes systemic and the bacteria appear primarily in the head kidney and spleen.When legal and sublegal individuals are compared there is a significantly higher level of splenic involvement, suggesting an age dependency. This is supported by a lower condition factor among legal fish as compared with sublegal, and the higher incidence of fish with both granulomas and sores among the legal sized striped bass. If comparisons are considered between the NGNS, GNS, and GS categories the trend is toward a significantly lower condition factor in the GS category as compared with NGNS and GNS categories. In legal fish this difference is not as apparent as in the sublegal fish.
It is clear from this work that bioenergetics is a useful tool in identifying changes in predator prey relationships, particularly when adequate benchmarks are available. Comparisons of results from the study by Hartman (1993) and those from this study indicate changes. Limitations do exist, particularly in addressing populations with extraordinary rates of infection by bacterial pathogens. Healthy fish were defined by the absence of granulomas and sores should be used for final assessments. The bioenergetics model used in this study did not take infections into account. Future analyses will be conducted to identify changes in the growth rates based on age data.The age data, when coupled with infection intensity may indicate how Mycobacterium sp. alter growth rates. This information will help eliminate the individuals used as part of the data base for the bioenergetics model.
The usefulness of diet studies, which are the basis of the bioenergetics model is underscored by the fact that if there has been a shift in prey for the striped bass populations resident to the Chesapeake Bay, then logically there will have been shifts in the types of micro and macro nutrients available.It is strongly suggested that emaciation among striped bass is because of the disease process, the condition(s) which result in an increased predilection of striped bass for becoming infected with Mycobacterium sp. may lie with altered diets. It is well know that nutritional shifts have deleterious effects on fish, and in many situations affect the immune status (Steffens 1989). This is not to say that this is the only possible cause for the condition, but is one of many possible origins of ulcerative dermatitis in striped bass.
Our assumption that the disease is progressive ( Figure 15) is in part based on the conceptual model of granulomatous inflammation by Adams (1983) and modified by Sindermann (1999) which suggests that there are three possible outcomes; resolution, mature granulomas, or death (Sindermann 1999).From our data and the model presented by Sindermann (1999), we would suggest that mycobacterial infections of striped bass begin in the Chesapeake Bay estuary and from the moment of infection progresses until death occurs in a large portion of the resident striped bass. From the fish taken during some of the age 1 and many of the age 2 fish were confirmed to have mycobacterial infections. Older fish were infected, and scale analyses will show how many of these fish were infected and to what intensity.
The ultimate repercussions of the disease process caused by Mycobacterium sp. and diet shifts on striped bass populations can only be inferred at this time. The disease process may be initiated at different ages in the striped bass, hence the life span after infection will differ. This would result in a slow cropping of infected individuals which die and are scavenged or debilitated and easily eaten by predators. In either situation mass mortalities would not be more obvious.The influence of mycobacterial infections should be seen in recruitment, with natural mortality rates greatly increased and survival to reproductive age decreased. Dietary shifts may have much broader effects with changes in prey species abundance, lack of proper development in young striped bass, lack of proper gonadal development, and increased susceptibility to a broad array of viral, bacterial, fungal, or metazoan pathogens.
It seems that the shifts in the feeding habits of striped bass have occurred since 1993.This coincides with a steady increase in the striped bass abundance and slow decrease in the abundance of traditional prey items. However it is not clear whether these shifts are due to the increase in demand and consumption by striped bass, environmental changes, or by anthropogenic factors. The repercussions of these shifts may be to adversely affect the health of the striped bass by limiting the amount of energy available through this tropic pathway. Alternatively, the effect may be by a direct increase in the consumption of the other fish species impacting both striped bass and other fish production in the Chesapeake Bay.We do show evidence of an increase in the occurrence of external lesions in the striped bass population and also an increase in bacteria, which seems to cause a decrease in the overall health of striped bass. It is not clear whether the changes in the feeding habits of striped bass have caused individuals to become more susceptible to certain bacteria, but we hypothesize that there is a strong correlation between the two.