Thomas E. Helser

Growth of the Atlantic surfclam, Spisula solidissima, from Georges Bank to the Delmarva Peninsula, USA

Age/shell length data for offshore surfclam, Spisula solidissima (Dillwyn, 1817), populations were used to estimate the parameters of the von Bertalanffy growth model by time period and region, from Georges Bank in the north to the Delmarva Peninsula in the south. Randomization tests were used to compare curves. We tested the a priori hypothesis that growth curves would change over time in the “south” (i.e., New Jersey and Delmarva) but remain constant in the “north” (i.e., Long Island and South New England). This hypothesis was proposed because surfclam population structure in the “south” had been altered by the hypoxic event of 1976, and possibly by intense, long-term commercial harvesting. Northern regions, unaffected by these factors, served as natural controls. Based on a comparison of data collected in 1980 with pooled data from 1989 and 1992, the hypothesis was supported. Both the growth coefficient (k) and maximum shell length (L∞) declined between two time periods in the two “southern” regions, while during the same time interval, no change occurred in the two “northern” regions. Differences in growth between regions were often statistically significant. For example, compared with the “southern” regions, the growth coefficient on Georges Bank was larger, and those clams attained a smaller maximum length. In a comparison of adjacent regions from Delmarva to S. New England, k increased from south to north. This could imply faster growth in cooler water, as well as no relationship between growth and primary productivity. Alternatively, size-selective mortality, imposed by the commercial fishery, was discussed as a mechanism that might account for this unexpected pattern.

Effect of Three Bycatch Reduction Devices on Diamondback Terrapin Malaclemys terrapin Capture and Blue Crab Callinectes sapidus Harvest in Delaware Bay

Abstract In recent years, reduction of the taking of nontarget species in fisheries worldwide has received considerable attention. Bycatch mortality in pot fisheries has led to severe declines in local populations of the diamondback terrapin Malaclemys terrapin along the U.S. East Coast. We conducted a 4-year study between 1997 and 2000 to investigate the impact of four bycatch reduction devices (BRDs), wire rectangular devices fitted into the entrance of crab traps, on diamondback terrapin bycatch mortality as well as its effect on the harvest of blue crabs Callinectes sapidus in the Delaware River estuary. A total of 52 sets consisting of five replicate blue crab pots for each BRD treatment (5.0 × 10.0, 5.0 × 12.0, 4.5 × 12.0, and 3.8 × 12.0 cm) and controls were deployed at random in 10 tidal tributaries and inland bays within the Delaware estuary. During this study 372 diamondback terrapins and 3,522 legal-size blue crabs were caught. The 5-cm × 12-cm BRD was the only treatment for which the reduction...

Otolith oxygen isotopes measured by high‐precision secondary ion mass spectrometry reflect life history of a yellowfin sole (Limanda aspera)

RATIONALE The oxygen isotope ratio (δ(18)O value) of aragonite fish otoliths is dependent on the temperature and the δ(18)O value of the ambient water and can thus reflect the environmental history of a fish. Secondary ion mass spectrometry (SIMS) offers a spatial-resolution advantage over conventional acid-digestion techniques for stable isotope analysis of otoliths, especially given their compact nature. METHODS High-precision otolith δ(18)O analysis was conducted with an IMS-1280 ion microprobe to investigate the life history of a yellowfin sole (Limanda aspera), a Bering Sea species known to migrate ontogenetically. The otolith was cut transversely through its core and one half was roasted to eliminate organic contaminants. Values of δ(18)O were measured in 10-µm spots along three transects (two in the roasted half, one in the unroasted half) from the core toward the edge. Otolith annual growth zones were dated using the dendrochronology technique of crossdating. RESULTS Measured values of δ(18)O ranged from 29.0 to 34.1‰ (relative to Vienna Standard Mean Ocean Water). Ontogenetic migration from shallow to deeper waters was reflected in generally increasing δ(18)O values from age-0 to approximately age-7 and subsequent stabilization after the expected onset of maturity at age-7. Cyclical variations of δ(18)O values within juvenile otolith growth zones, up to 3.9‰ in magnitude, were caused by a combination of seasonal changes in the temperature and the δ(18)O value of the ambient water. CONCLUSIONS The ion microprobe produced a high-precision and high-resolution record of the relative environmental conditions experienced by a yellowfin sole that was consistent with population-level studies of ontogeny. Furthermore, this study represents the first time that crossdating has been used to ensure the dating accuracy of δ(18)O measurements in otoliths.

In situ reproduction, abundance, and growth of young‐of‐year and adult largemouth bass in a population exposed to polychlorinated biphenyls

We conducted a two-year field study (2000-2001) in the Housatonic River, Massachusetts (USA) to determine if we could detect in situ population-level effects on largemouth bass (Micropterus salmoides) exposed to elevated levels of polychlorinated biphenyls (PCBs). Calculated whole-body PCB concentrations in adult bass in 2002 averaged 121 mg/kg (range = 34-556 mg/kg). Polychlorinated biphenyl concentrations in young-of-year (YOY) composites in 2000 and 2002 averaged 28 mg/kg (range = 21-41 mg/kg) and 19 mg/kg (range = 16-24 mg/kg), respectively. Laboratory studies of fish have reported PCB toxicity at exposure levels below and within the range of those found in the Housatonic River. We evaluated five field-derived metrics: reproductive activity, relative abundance of YOY, YOY growth rates, adult growth, and adult condition to determine whether we could detect effects of PCBs in the largemouth bass population. These computed metrics, when compared with data sets assembled for numerous largemouth bass populations in North America, provided no evidence of population-level impairment. Results of this study suggest that PCB tissue concentrations associated with effects in laboratory studies do not necessarily translate to detectable effects on largemouth bass populations in their natural environment.

An Age-Structured Bioeconomic Simulation of U.S. Silver Hake Fisheries

Abstract We present a bioeconomic simulation of the U.S. fisheries for silver hake Merluccius bilinearis, an abundant species distributed over the northwest Atlantic continental shelf and historically important to both U.S. and foreign fishing fleets. The model combines elements of agestructured population and harvest yield models with a two-equation price response model. The analysis evaluates biological benefits of interest to managers, such as future yields or rebuilding of parental stock, as well as future revenues and net present value of interest to harvesters. In one set of simulations, yield and revenue response surfaces were generated for varying levels of fishing mortality (F) and selection at age under constant annual recruitment. In another, a stochastic stock–recruitment function permitted assessment of yield and revenue trajectories over time given variable annual recruitment. Under equilibrium conditions, response surfaces for both total fishery yield and revenue are asymptotic with increas...

Reproductive Characteristics of Weakfish in Delaware Bay: Implications for Management

Abstract We quantified size and age at maturity, batch fecundity, spawning frequency, and annual fecundity for weakfish Cynoscion regalis in Delaware Bay in 1999 and 2000. Information about these critical characteristics are needed for stock assessment models, which have never been assessed in Delaware Bay nor anywhere in the range of the weakfish since 1991–1992, when abundance was very low. Stock size increased from 1991–1992 to 1999–2000, but size at maturity was 168 ± 4.1 mm (mean ± SE) and did not appear to increase with stock size. Ninety-seven percent of age-1 fish were mature, but they arrived later to the estuary, had lower batch fecundity, and spawned less frequently than did older fish. The annual fecundity of fish age 2–6 was at least 30 times as great as that of age-1 fish. Batch fecundity was similar between Chesapeake Bay and Delaware Bay in the same year once corrected for fish size. The relationships of batch fecundity to both female weight and length were statistically equal between the ...

Evaluation of micromilling/conventional isotope ratio mass spectrometry and secondary ion mass spectrometry of δ18O values in fish otoliths for sclerochronology

RATIONALE Stable oxygen isotope ratios (δ18 O values) measured in fish otoliths can provide valuable detailed information on fish life history, fish age determination, and ocean thermography. Traditionally, otoliths are sampled by micromilling followed by isotope ratio mass spectrometry (IRMS), but direct analysis by secondary ion mass spectrometry (SIMS) is becoming more common. However, these two methods have not been compared to determine which, if either, is better for fish age validation studies. Hence, the goals were to: (1) determine if the δ18 O signatures from the two different methods are similar, (2) determine which method is better for fish age validation studies, and (3) examine biogeographic and migration history. METHODS Both analytical techniques, micromilling/IRMS and SIMS, were used to measure δ18 O values in six Pacific cod (Gadus macrocephalus) otoliths. A series of measurements was made from the center of each otolith to its edge to develop a life-history δ18 O signature for each fish. RESULTS The sampling resolution of SIMS analyses was 2-3 times greater than that obtained by micromilling/IRMS. We found an offset between SIMS and micromilling/IRMS δ18 O values, about 0.5‰ on average, with SIMS yielding lower values. However, the δ18 O patterns from both methods (i.e., the number of δ18 O maxima) correspond to the estimated age determined by otolith growth-zone counts, validating fish age determination methods. CONCLUSIONS Both techniques resolved δ18 O life-history signatures and showed patterns consistent with seasonal variation in temperatures and changes due to fish migration. When otoliths are large, micromilling/IRMS can provide adequate resolution for fish age validation. However, SIMS is the better option if greater sampling resolution is required, such as when otoliths are small or specimens are longer lived and have compact growth zones.

Intrinsic and environmental drivers of growth in an Alaskan rockfish: an otolith biochronology approach

Otolith growth-increment chronologies provide an approach for evaluating the impacts of both high-frequency (e.g., interannual) and low-frequency (e.g., interdecadal) climate variability on fish growth. A growth-increment biochronology spanning six decades, spanning several warm and cold climate regime periods, was developed for a commercially important species of rockfish, Sebastes polyspinis, in the Gulf of Alaska. To confirm that all increments were correctly identified and placed in time, we borrowed the technique of crossdating from the tree-ring science of dendrochronology, which ensured high data quality. We then used a mixed effects model to partition variance in otolith growth-increment width among intrinsic (e.g., age-related) and extrinsic (e.g., climate-related) factors. This biochronology was contrasted with one recently developed for S. alutus, a closely-related species which exhibited a significant change in growth following the late 1970s North Pacific climate regime shift. Both species generally showed positive relationships between warm climate conditions and growth, though S. polyspinis experienced a relatively smaller step-increase in growth following the regime shift. The new S. polyspinis otolith biochronology represents a long-term record of growth that extends well before biological specimens were first collected in the Gulf of Alaska, providing a potential tool for fisheries managers to evaluate the effects of climate variability on growth and biological reference points.