Red snapper (Lutjanus campechanus) demographic structure in the northern Gulf of Mexico based on spatial patterns in growth rates and morphometrics

Red snapper (Lutjanus campechanus) in the United States waters of the Gulf of Mexico (GOM) has been considered a single unit stock since management of the species began in 1991. The validity of this assumption is essential to management decisions because measures of growth can differ for nonmixing populations. We examined growth rates, size-at-age, and length and weight information of red snapper collected from the recreational harvests of Alabama (n=2010), Louisiana (n=1905), and Texas (n =1277) from 1999 to 2001. Ages were obtained from 5035 otolith sections and ranged from one to 45 years. Fork length, total weight, and age-frequency distributions differed significantly among all states; Texas, however, had a much higher proportion of smaller, younger fish. All red snapper showed rapid growth until about age 10 years, after which growth slowed considerably. Von Bertalanffy growth models of both mean fork length and mean total weight-at-age predicted significantly smaller fish at age from Texas, whereas no differences were found between Alabama and Louisiana models. Texas red snapper were also shown to differ significantly from both Alabama and Louisiana red snapper in regressions of mean weight at age. Demographic variation in growth rates may indicate the existence of separate management units of red snapper in the GOM. Our data indicate that the red snapper inhabiting the waters off Texas are reaching smaller maximum sizes at a faster rate and have a consistently smaller total weight at age than those collected from Louisiana and Alabama waters. Whether these differences are environmentally induced or are the result of genetic divergence remains to be determined, but they should be considered for future management regulations.

Capture probability of a survey trawl for red king crab (Paralithodes camtschaticus)

The relative abundance of Bristol Bay red king crab (Paralithodes camtschaticus) is estimated each year for stock assessment by using catch-per-swept-area data collected on the Alaska Fisheries Science Center’s annual eastern Bering Sea bottom trawl survey. To estimate survey trawl capture efficiency for red king crab, an experiment was conducted with an auxiliary net (fitted with its own heavy chain-link footrope) that was attached beneath the trawl to capture crabs escaping under the survey trawl footrope. Capture probability was then estimated by fitting a model to the proportion of crabs captured and crab size data. For males, mean capture probability was 72% at 95 mm (carapace length), the size at which full vulnerability to the survey trawl is assigned in the current management model; 84.1% at 135 mm, the legal size for the fishery; and 93% at 184 mm, the maximum size observed in this study. For females, mean capture probability was 70% at 90 mm, the size at which full vulnerability to the survey trawl is assigned in the current management model, and 77% at 162 mm, the maximum size observed in this study. The precision of our estimates for each sex decreased for juveniles under 60 mm and for the largest crab because of small sample sizes. In situ data collected from trawl-mounted video cameras were used to determine the importance of various factors associated with the capture of individual crabs. Capture probability was significantly higher when a crab was standing when struck by the footrope, rather than crouching, and higher when a crab was hit along its body axis, rather than from the side. Capture probability also increased as a function of increasing crab size but decreased with increasing footrope distance from the bottom and when artificial light was provided for the video camera.

Estimation of bycatch in shrimp trawl fisheries: a comparison of estimation methods using field data and simulated data

Bycatch, or the incidental catch of nontarget organisms during fi shing operations, is a major issue in U.S. shrimp trawl fisheries. Because bycatch is typically discarded at sea, total bycatch is usually estimated by extrapolating from an observed bycatch sample to the entire fleet with either mean-per-unit or ratio estimators. Using both field observations of commercial shrimp trawlers and computer simulations, I compared five methods for generating bycatch estimates that were used in past studies, a mean-per-unit estimator and four forms of the ratio estimator, respectively: 1) the mean fish catch per unit of effort, where unit effort was a proxy for sample size, 2) the mean of the individual fish to shrimp ratios, 3) the ratio of mean fish catch to mean shrimp catch, 4) the mean of the ratios of fish catch per time fished (a variable measure of effort), and 5) the ratio of mean fish catch per mean time fished. For field data, different methods used to estimate bycatch of Atlantic croaker, spot, and weakfish yielded extremely different results, with no discernible pattern in the estimates by method, geographic region, or species. Simulated fishing fleets were used to compare bycatch estimated by the fi ve methods with “actual” (simulated) bycatch. Simulations were conducted by using both normal and delta lognormal distributions of fish and shrimp and employed a range of values for several parameters, including mean catches of fish and shrimp, variability in the catches of fish and shrimp, variability in fishing effort, number of observations, and correlations between fish and shrimp catches. Results indicated that only the mean per unit estimators provided statistically unbiased estimates, while all other methods overestimated bycatch. The mean of the individual fish to shrimp ratios, the method used in the South Atlantic Bight before the 1990s, gave the most biased estimates. Because of the statistically significant two- and 3-way interactions among parameters, it is unlikely that estimates generated by one method can be converted or corrected to estimates made by another method: therefore bycatch estimates obtained with different methods should not be compared directly.

Modeling red sea urchin (Strongylocentrotus franciscanus) growth using six growth functions

The growth of red sea urchins (Strongylocentrotus franciscanus) was modeled by using tag-recapture data from northern California. Red sea urchins (n=211) ranging in test diameter from 7 to 131 mm were examined for changes in size over one year. We used the function Jt+1 = Jt + f(Jt) to model growth, in which Jt is the jaw size (mm) at tagging, and Jt+1 is the jaw size one year later. The function f(Jt), represents one of six deterministic models: logistic dose response, Gaussian, Tanaka, Ricker, Richards, and von Bertalanffy with 3, 3, 3, 2, 3, and 2 minimization parameters, respectively. We found that three measures of goodness of fi t ranked the models similarly, in the order given. The results from these six models indicate that red sea urchins are slow growing animals (mean of 7.2 ±1.3 years to enter the fishery). We show that poor model selection or data from a limited range of urchin sizes (or both) produces erroneous growth parameter estimates and years-to-fishery estimates. Individual variation in growth dominated spatial variation at shallow and deep sites (F=0.246, n=199, P=0.62). We summarize the six models using a composite growth curve of jaw size, J, as a function of time, t: J = A(B – e–Ct) + Dt, in which each model is distinguished by the constants A, B, C, and D. We suggest that this composite model has the flexibility of the other six models and could be broadly applied. Given the robustness of our results regarding the number of years to enter the fishery, this information could be incorporated into future fishery management plans for red sea urchins in northern California.

Bycatch of lined seahorses (Hippocampus erectus) in a Gulf of Mexico shrimp trawl fishery

Bycatch studies have largely ignored population level effects on fish species of little commercial interest. Here we analyze bycatch of the lined seahorse (Hippocampus erectus) in the bait-shrimp trawl fishery in Hernando Beach, Florida, providing the first fisheries data for this species. Based on catch per unit of effort (CPUE), size, sex, and reproductive status of trawled H. erectus, 1) approximately 72,000 seahorses were caught annually by this fleet, from a population of unknown size, 2) trawling affected population cohorts differentially because of temporal and spatial variation in CPUE and population size, and 3) a greater proportion of females than males was removed in trawling. Our findings suggest that trawling may affect seahorse populations through direct mortality, social disruption, and habitat damage. However, the lack of specific abundance or catchability estimates for H. erectus means that the precise impact of trawling on this fish remains uncertain. This paper focuses attention on the need for research and monitoring of small fishes that are caught incidentally in nonselective gear.

Tag-reporting levels for red drum (Sciaenops ocellatus) caught by anglers in South Carolina and Georgia estuaries

A total of 1784 legal-size (≥356 mm TL) hatchery-produced red drum (Sciaenops ocellatus) were tagged and released to estimate tag-reporting levels of recreational anglers in South Carolina (SC) and Georgia (GA). Twelve groups of legal-size fish (~150 fish/group) were released. Half of the fish of each group were tagged with an external tag with the message “reward” and the other half of the fish were implanted with tags with the message “$100 reward.” These fish were released into two estuaries in each state (n=4); three replicate groups were released at different sites within each estuary (n=12). From results obtained in previous tag return experiments conducted by wildlife and fisheries biologists, it was hypothesized that reporting would be maximized at a reward level of $100/tag. Reporting level for the “reward” tags was estimated by dividing the number of “reward” tags returned by the number of “$100 reward” tags returned. The cumulative return level for both tag messages was 22.7 (±1.9)% in SC and 25.8 (±4.1)% in GA. These return levels were typical of those recorded by other red drum tagging programs in the region. Return data were partitioned according to verbal survey information obtained from anglers who reported tagged fish. Based on this partitioned data set, 14.3 (±2.1)% of “reward” tags were returned in SC, and 25.5 (±2.3)% of “$100 reward” tags were returned. This finding indicates that only 56.7% of the fish captured with “reward” tags were reported in SC. The pattern was similar for GA where 19.1 (±10.6)% of “reward” message tags were returned as compared with 30.1 (±15.6)% for “$100 reward” message tags. This difference yielded a reporting level of 63% for “reward” tags in GA. Currently, 50% is used as the estimate for the angler reporting level in population models for red drum and a number of other coastal finfish species in the South Atlantic region of the United States. Based on results of our study, the commonly used reporting estimate may result in an overestimate of angler exploitation for red drum.

Estimated ages of red porgy (Pagrus pagrus) from fishery-dependent and fishery-independent data and a comparison of growth parameters

The red porgy, Pagrus pagrus, is an important reef fish in several offshore fisheries along the southeastern United States. We examined samples from North Carolina through southeast Florida from recreational (headboat) and commercial (hook and line) fisheries, as well as samples from a fishery-independent source. Red porgy attain a maximum age of at least 18 years and 733 mm total length. The weight-length relationship is represented by the ln-ln transformed equation: W = 8.85 × 10–6(L)3.06, where W = whole weight in grams, and L = total length in mm. The von Bertalanffy growth equation fitted to the most recent, back-calculated lengths from all the samples is Lt = 644(1 – e –0.15(t + 0.76)). Our study revealed a difference in mean length at age of red porgy from the three sources. Red porgy in fishery-independent collections were smaller at age than specimens examined from fishery-dependent sources. The difference in length-at-age may be related to gear selectivity and have important consequences in the assessment of fish stocks.

A new growth model for red drum (Sciaenops ocellatus) that accommodates seasonal and ontogenic changes in growth rates

The red drum (Sciaenops ocellatus) is a popular gamefish found throughout the coastal waters of the Gulf of Mexico and along the eastern seaboard as far north as Massachusetts. Juvenile red drum grow extremely rapidly, especially during the warmer months, but adults grow very little. In fact, the change in growth with age is so abrupt that the standard von Bertalanffy curve has proven inadequate— the predicted lengths of younger fish are generally too large and the predicted lengths of older fish too small (see Beckman et al., 1988; Murphy and Taylor, 1990).

Efficiency of bycatch reduction devices in small otter trawls used in the Florida shrimp fishery

Two bycatch reduction devices (BRDs)—the extended mesh funnel (EMF) and the Florida fisheye (FFE)—were evaluated in otter trawls with net mouth circumferences of 14 m, 17 m, and 20 m and total net areas of 45 m2. Each test net was towed 20 times in parallel with a control net that had the same dimensions and configuration but no BRD. Both BRDs were tested at night during fall 1996 and winter 1997 in Tampa Bay, Florida. Usually, the bycatch was composed principally of finfish (44 species were captured); horseshoe crabs and blue crabs seasonally predominated in some trawls. Ten finfish species composed 92% of the total finfish catch; commercially or recreationally valuable species accounted for 7% of the catch. Mean finfish size in the BRD-equipped nets was usually slightly smaller than that in the control nets. Compared with the corresponding control nets, both biomass and number of finfish were almost always less in the BRD-equipped nets but neither shrimp number nor biomass were significantly reduced. The differences in proportions of both shrimp and finfish catch between the BRD-equipped and control nets varied between seasons and among net sizes, and differences in finfish catch were specific for each BRD type and season. In winter, shrimp catch was highest and size range of shrimp was greater than in fall. Season-specific differences in shrimp catch among the BRD types occurred only in the 14-m, EMF nets. Finfish bycatch species composition was also highly seasonal; each species was captured mainly during only one season. However, regardless of the finfish composition, the shrimp catch was relatively constant. In part as a result of this study, the State of Florida now requires the use of BRDs in state waters.

Severe decline in abundance of the red porgy (Pagrus pagrus) population off the southeastern United States

Status of the southeastern U.S. stock of red porgy (Pagrus pagrus) was estimated from fishery-dependent and fishery-independent data, 1972–97. Annual population numbers and fishing mortality rates at age were estimated from virtual population analysis (VPA) calibrated with fishery-independent data. For the VPA, a primary matrix of catch at age was based on age-length keys from fishery-independent samples; an alternate matrix was based on fishery-dependent keys. Additional estimates of stock status were obtained from a surplus-production model, also calibrated with fishery-independent indices of abundance. Results describe a dramatic increase in exploitation of this stock and concomitant decline in abundance. Estimated fully recruited fishing mortality rate (F) from the primary catch matrix increased from 0.10/yr in 1975 to 0.88/yr in 1997, and estimated static spawning potential ratio (SPR) declined from about 67% to about 18%. Estimated recruitment to age 1 declined from a peak of 3.0 million fish in 1973–74 to 94,000 fish in 1997, a decline of 96.9%. Estimated spawning-stock biomass declined from a peak of 3530 t in 1979 to 397 t in 1997, a decline of 88.8%. Results from the alternate catch matrix were similar. Retrospective patterns in the VPA suggest that the future estimates of this population decline will be severe, but may be less than present estimates. Long-term and marked declines in recruitment, spawning stock, and catch per unit of effort (both fishery-derived and fishery-independent)are consistent with severe overexploitation during a period of reduced recruitment. Although F prior to 1995 has generally been estimated at or below the current management criterion for overfishing (F equivalent to SPR=35%), the recent spawning-stock biomass is well below the biomass that could support maximum sustainable yield. Significant reductions in fishing mortality will be needed for rebuilding the southeastern U.S. stock.

Effects of experimental harvest on red sea urchins (Strongylocentrotus franciscanus) in northern Washington

Commercial harvest of red sea urchins began in Washington state in 1971. Harvests peaked in the late 1980s and have since declined substantially in Washington and other areas of the U.S. west coast. We studied effects of experimental harvest on red sea urchins in San Juan Channel (SJC), a marine reserve in northern Washing-ton. We recorded changes in density and size distribution of sea urchin populations resulting from three levels of experimental harvest: 1) annual size-selective harvest (simulating cur-rent commercial urchin harvest regulations), 2) monthly complete (non–size selective) harvest, and 3) no harvest (control) sites. We also examined re-colonization rates of harvested sites. The red sea urchin population in SJC is composed of an accumulation of large, old individuals. Juvenile urchins represent less than 1% of the population. Lower and upper size limits for commercial harvest protect 5% and 45% of the population, respectively. Complete harvest reduced sea urchin densities by 95%. Annual size-selective harvest significantly decreased sea urchin densities by 67% in the first year and by 47% in the second year. Two years of size-selective harvest significantly altered the size distribution of urchins, decreasing the density of legal-size urchins. Recolonization of harvested sites varied seasonally and occurred primarily through immigration of adults. Selective harvest sites were recolonized to 51% and 38% of original densities, respectively, six months after the first and second annual harvests. Yields declined substantially in the second year of size-selective harvest because of the fishing down of the population and because of low recolonization rates of harvested sites. We recommend that managers consider the potential efficacy of marine harvest refuges and reevaluate the existing upper and lower size limits for commercial harvest to improve long-term management of the sea urchin fishery in Washington.

Sustainability of elasmobranchs caught as bycatch in a tropical prawn (shrimp) trawl fishery

The bycatch of Australia’s northern prawn fishery (NPF) comprises 56 elasmobranch species (16 families). The impact of this fishery on the sustainability of these species has not been addressed. We obtained estimates of catch rates and the within-net survival of elasmobranchs. Carcharhinus tilstoni, C. dussumieri, Rhynchobatus djiddensis, and Himantura toshi represented 65% of the bycatch. For most species, >50% of individuals in the bycatch were immature, and some species recruited to the fishery at birth. For all species combined, 66% of individuals in the bycatch died in the trawl net. The relative sustainability of elasmobranchs caught as bycatch was examined by ranking species with respect to their susceptibility to capture and mortality due to prawn trawling and with respect to their capacity to recover once the population was depleted. The species that were least likely to be sustainable were four species of pristids, Dasyatis brevicaudata, and Himantura jenkinsii. These are bottom-associated batoids that feed on benthic organisms and are highly susceptible to capture in prawn trawls. The recovery capacity of these species was also low according to our criteria. Our results provide a valuable first step towards ensuring the sustainability of elasmobranchs that are caught as bycatch by highlighting species for management and research. The effectiveness of turtle excluder devices (TEDs) in reducing elasmobranch bycatch varied greatly among species but was generally not very effective because most of the captured species were small.