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Spanish mackerel were purchased from commercial fisheries in the Chesapeake Bay in the period 1993-1994 and processed for biological data to describe reproductive biology. Spanish mackerel are multiple spawners with asynchronous oocyte development and indeterminate fecundity. They spawn in the Chesapeake Bay area from June through August, June being the peak spawning month. Individuals, however, vary greatly in when they begin and end spawning. The presence of Gravid and Running Ripe females indicates spawning inside the Chesapeake Bay, and the capture of small juveniles suggests the region may be used as a nursery grounds. Sex ratios vary greatly, females generally dominating at larger sizes. Length at first maturity (L50 ) was 230 mm FL in females and 232 mm FL in males; all females are mature at 330 mm FL and all males at 340 mm FL. Mean batch fecundity is 74,077 eggs, increasing with length and somatic weight. Average relative fecundity is 143 eggs/g female(± 10 SE). Spanish mackerel are .sometimes infected with a live-bearing nematode that reduces fecundity.

No description available.

No description available.

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Stable isotope techniques were applied to summer flounder, Paralichthys dentatus, in Chesapeake Bay to elucidate the relative importance of different prey groups on the growth and productivity of this species. Prior to field application, a laboratory diet-shift study was conducted to evaluate methodological assumptions and obtain necessary isotopic parameters. Specifically, the goals of the laboratory study were to 1) determine isotopic turnover rates and fractionations of δ13C and δ15N in liver, whole blood, and white muscle and 2) estimate the relative importance of growth and metabolic processes on isotopic turnover. Groups of captive juvenile summer flounder (130-255mm total length) were monitored for up to 180 days after switching their food to a new diet with different stable isotope values. Although differences existed between carbon (C) and nitrogen (N), the rate of isotopic change was consistently ranked liver>blood>muscle for the three tissues due to increased metabolic activities of liver and blood. Half lives ranged from 9-21, 20-44, 49-73 days for liver, blood, and muscle respectively. Fractionation estimates for δ15N in muscle (range: 2.4-4.2‰) corresponded with previous research, but estimates for δ13C (range: 0.1-4.8‰) tended to be greater than the traditionally assumed values of 0-1‰. Liver and blood fractionation estimates were similar to those of muscle, differing by usually <1‰. A generalized model for predicting the time scale of isotopic turnover from growth-based turnover parameters was also developed to help evaluate assumptions of isotopic equilibrium in the field. Information obtained from the laboratory study facilitated the use of stable isotopes as dietary tracers for wild summer flounder (138-624mm total length) in Chesapeake Bay. Summer flounder tissues (liver, blood, and muscle) and commonly consumed prey species were sampled seasonally during late spring / early summer (May-July) and fall (November) in 2006 and 2007. To account for similarity in isotopic measurements and to apply mixing models, prey species were aggregated into two trophic guilds: crustaceans (mysid shrimp, sand shrimp, mantis shrimp) and fishes (bay anchovy, juvenile sciaenids, spotted hake). Lack of δ13C differentiation among trophic guilds and summer flounder prevented the use of δ13C as a useful dietary indicator. Analysis of δ15N revealed that crustaceans comprised the majority of summer flounder diet, accounting for ~85-100% of flounder diets on average, except in spring of 2006 when fishes and crustaceans were equally represented in the diet. Summer flounder tended to occupy the same trophic level as the other fishes, suggesting more of a competitive relationship than a predatory one. However, a positive trend in δ15N with length in all tissues indicated that larger summer flounder fed at ~1 trophic level above smaller flounder. Differences in isotopic values between slow and fast turnover tissues did not reveal this ontogenetic dietary pattern at the level of the individual, because the changes in feeding were of small isotopic magnitude and occurred too gradually for reliable detection. Based on stable isotopic analysis, growth and production of summer flounder in Chesapeake Bay are highly dependent on assimilation of mysid, sand, and mantis shrimps, more so than previously expected based on stomach content research.

Otoliths, scales, dorsal spines, and pectoral fin rays were compared to determine the best hardpart for ageing weakfish, Cynoscion regalis. Sectioned otoliths showed the clearest marks and were validated by the marginal increment method for ages 1-5. Traditionally-used scales were found to be less-precise and to underage older fish. Most weakfish from the Chesapeake Bay region were 200-600 mm TL and ages 1-4. Weakfish were not fully-recruited to commercial foodfish grades until age 2. Maximum observed age was 17 from a Delaware Bay fish collected in 1985. Current maximum observed ages were age 12 in Chesapeake Bay and age 11 in Delaware Bay. Fish older than age 6 were rare in both areas. Weakfish growth was well-described by the von Bertalanffy growth model (R&\sp2& = 0.98, N = 857). There was no evidence Delaware Bay weakfish reached a larger asymptotic length or size-at-age than Chesapeake Bay fish. However, maximum size and age from both areas has fluctuated in the past thirty years. Maximum size greatly increased from the late 1960's until roughly 1985, as did the numbers of large fish, apparently due to a series of strong year-classes, beginning in the late 1960's. Weakfish are multiple spawners with indeterminate fecundity and a spawning season from May to August, in the Chesapeake Bay region. Sex ratios were approximately 3:1, females to males, in 1990-1992. Mean length at first maturity for males and females was 164 and 170 mm TL, respectively. Most fish were mature by age one and all fish were mature by age 2. Spawning activity was not consistent throughout the spawning season or between 1991 and 1992. Batch fecundities ranged from 75,289-517,845 eggs/female and significantly increased with both TL and somatic weight. Spawning frequency was every 2-3 days in 1991 and every 12-13 days in 1992, leading to an average annual fecundity of 7,369,750 eggs/females in 1991 and 1,808,056 eggs/female in 1992. Patterns of spawning activity within and between years appeared closely associated with feeding success. Yield-per-recruit analysis indicated that, over a likely range of natural mortality rates, growth overfishing is currently occurring in the Chesapeake Bay region. Current t&\sb{lcub}\rm c{rcub}& is &\le& age 2, whereas maximum yield consistently occurred at t&\sb{lcub}\rm c{rcub}& &\ge& age 6.

Despite an increased global interest in harmful algal bloom (HAB) species and eutrophication, the relationship between nutrient sources and changes in species composition or toxicity remains unclear. Stable isotopes are routinely used to identify and track nitrogen (N) sources to water bodies, as sources can be differentiated based on stable isotope values. While literature is available describing N fractionation by diatoms and coccolithophores, data are greatly lacking regarding isotope fractionation by dinoflagellates. Here we investigate the fractionation of nitrogen isotopes by saxitoxin-producing Alexandrium fundyense, to validate the use of the δ15N of particulate organic matter and identify the nitrogen source fueling a dinoflagellate bloom and its toxicity. The effects of N chemical form on isotope fractionation, toxin content, and toxicity, were investigated using isolates in single-N and mixed-N experiments. Growth on NO3-, NH4+, or urea, resulted in isotope fractionation of 2.761.48‰, 29.019.32‰, or 0.340.19‰, respectively, with the lowest cellular toxicity and toxin quotas reported during urea utilization. Toxin composition and growth rates, however, remained constant across all N treatments, showing no effects of NO3-, NH4+, or urea utilization. Alexandrium fundyense was then preconditioned to either NO3-, NH4+, or urea, and abruptly inoculated into mixed-N medium containing all three chemical forms. All treatments initially utilized NH4+ and urea upon inoculation into mixed medium, suggesting no effect of preconditioning. Cells only began utilizing NO3- after NH4+ decreased below 2-4 M in the medium. During the inhibition of NO3- uptake by NH4+ utilization, the cellular δ15N was at its lowest (-5‰), and through the course of the experiment, the δ15N continuously changed to mimic the isotope value of the most recent N source(s) being utilized. When utilizing multiple sources, the isotope signature of the cells fell between the signal of the two N sources. Together this suggests that in NO3- and urea rich environments, the 15NPOM would reliably look like the source or sources of nitrogen utilized, but that caution should be taken in NH4+ rich environments where the large value could lead to misinterpretation of the signal. Nutrients are only one factor influencing bloom dynamics, but information about the relative importance of natural or anthropogenic nutrients in the development and toxicity of bloom events is necessary to predict future shifts in phytoplankton species composition, density, and toxicity.