Robert T. Leaf

Elasticity Analyses Of Size-Based Red And White Abalone Matrix Models: Management And Conservation

Prospective elasticity analyses have been used to aid in the management of fished species and the conservation of endangered species. Elasticities were examined for deterministic size-based matrix models of red abalone, Haliotis rufescens, and white abalone, H. sorenseni, to evaluate which size classes influenced population growth (lambda) the most. In the red abalone matrix, growth transitions were determined from a tag recapture study and grouped into nine size classes. In the white abalone matrix, abalone growth was determined from a laboratory study and grouped into five size classes. Survivorship was estimated from tag recapture data for red abalone using a Jolly-Seber model with size as a covariate and used for both red and white abalone. Reproduction estimates for both models used averages of the number of mature eggs produced by female red and white abalone in each size class from four-year reproduction studies. Population growth rate (lambda) was set to 1.0, and the first-year survival (larval survival through to the first size class) was estimated by iteration. Survival elasticities were higher than fecundity elasticities in both the red and white matrix models. The sizes classes with the greatest survival elasticities, and therefore the most influence on population growth in the model, were the sublegal red abalone (150-178 mm) and the largest white abalone size class (140-175 mm). For red abalone, the existing minimum legal size (178 mm) protects the size class the model suggests is critical to population growth. Implementation of education programs for novice divers coupled with renewed enforcement may serve to minimize incidental mortality of the critical size class. For white abalone, conservation efforts directed at restoring adults may have more of an impact on population growth than efforts focusing on juveniles. Our work is an example of how prospective elasticity analyses of size-structured matrix models can be used to quantitatively evaluate research priorities, fishery management strategies, and conservation options.

Estimation of the Length-at-Age Relationship of Mississippi's Spotted Seatrout

AbstractSpotted Seatrout Cynoscion nebulosus are a highly valued recreational inshore species in Mississippi coastal waters. The accurate description of the length-at-age relationship is critical for assessment efforts of the stock. Because Spotted Seatrout exhibit small-scale movements, the stocks in the Gulf of Mexico are managed as state-specific units. Therefore, local demographic estimates of length at age are needed for assessment. We estimated the length-at-age relationship of Spotted Seatrout in Mississippi by using tag recapture records and otolith-derived age estimates. Three nonlinear length-at-age models were fit to sex-aggregated, tag recapture data and four nonlinear length-at-age models were fit to sex-specific, otolith-derived age data. For each suite of models, model support was determined using Akaike information criteria. The Francis (1988a) GROTAG method had the greatest support of the three models fit to the tag recapture data, and the resulting parameter estimates from the model were...

Bomb radiocarbon dating of the endangered white abalone (Haliotis sorenseni): investigations of age, growth and lifespan

Understanding basic life-history characteristics of white abalone (Haliotis sorenseni), such as estimated lifespan, is critical to making informed decisions regarding the recovery of this endangered species. All predictive modelling tools used to forecast the status and health of populations following restoration activities depend on a validated estimate of adult lifespan. Of the seven Haliotis species in California, white abalone is considered to have the highest extinctionrisk andwasthefirstmarineinvertebrate listedasanendangeredspeciesundertheFederalEndangeredSpecies Act (ESA). Lifespan was previously estimated from observations of early growth; however, no study has generated ages for the largest white abalone. To address questions of age and growth, bomb radiocarbon (D 14 C) dating was used on shells from large white abalone. Measured bomb D 14 C levels were compared to regional D 14 C reference records to provide estimates of age, growth and lifespan. Bomb radiocarbon dating indicated that growth was variable among individuals, with a maximum estimated age of 27 years. The findings presented here provide support for previous age and growth estimates and an estimated lifespan near 30 years. These age data support the perception of a critical need for restoringthe remnant aging and potentially senescent population. Additional keywords: age validation, carbon-14, Haliotidae, longevity, Mollusca, Southern California Bight.

A global perspective on the trophic geography of sharks

Sharks are a diverse group of mobile predators that forage across varied spatial scales and have the potential to influence food web dynamics. The ecological consequences of recent declines in shark biomass may extend across broader geographic ranges if shark taxa display common behavioural traits. By tracking the original site of photosynthetic fixation of carbon atoms that were ultimately assimilated into muscle tissues of 5,394 sharks from 114 species, we identify globally consistent biogeographic traits in trophic interactions between sharks found in different habitats. We show that populations of shelf-dwelling sharks derive a substantial proportion of their carbon from regional pelagic sources, but contain individuals that forage within additional isotopically diverse local food webs, such as those supported by terrestrial plant sources, benthic production and macrophytes. In contrast, oceanic sharks seem to use carbon derived from between 30° and 50° of latitude. Global-scale compilations of stable isotope data combined with biogeochemical modelling generate hypotheses regarding animal behaviours that can be tested with other methodological approaches.Carbon isotopic analysis reveals global biogeographic traits in shark trophic interactions, and sheds light on the diverse foraging behaviour of sharks.

THE FEASIBILITY OF BOMB RADIOCARBON ANALYSIS TO SUPPORT AN AGE-AT-LENGTH RELATIONSHIP FOR RED ABALONE, HALIOTIS RUFESCENS SWAINSON IN NORTHERN CALIFORNIA

Abstract Analysis of bomb generated radiocarbon (14C) changes in a red abalone, Haliotis rufescens Swainson shell was used to evaluate age-at-length relationships derived from data from a previous multiyear, multisite tag-recapture study. Shell carbonate was extracted from four successive growth trajectory locations in a single shell with a maximum shell length of 251 mm. Extraction locations were based on Von Bertalanffy growth function (VBGF) predictions and chosen to span the initial rise of the 14C-bomb pulse that is known to have occurred in surface ocean waters during 1958 ±1 y in the northeast Pacific. The close temporal correspondence of the red abalone sample series to regional Δ14C records demonstrated the utility of the technique for validating age-at-length relationships for the red abalone. The findings provided support for a mean VBGF derived age of 32 y (range 30–33 y) for the specimen; however, the analysis of 14C data indicated that the specimen could be older.

Growth and Spawning Dynamics of Southern Flounder in the North-Central Gulf of Mexico

AbstractSouthern Flounder Paralichthys lethostigma is the most commonly harvested flatfish in the north-central Gulf of Mexico (GOM), yet knowledge of this species’ growth and reproduction is limited for the Mississippi region. Given the offshore spawning migrations and likely mixing between state-managed units of the GOM Southern Flounder stock, small-scale environmental influences may constitute a major driver of life history dynamics during early life residency in nearshore estuaries. Therefore, estimates of local demographic characteristics are needed for effective fishery management. Here, we describe critical life history traits of Southern Flounder, including estimation of growth rates and maturity. The three-parameter von Bertalanffy growth function best described female-specific growth: mean asymptotic length L∞ was 514 mm TL, mean growth coefficient k was 0.67 year–1, and mean theoretical age at zero length t0 was –0.50 years. The estimated female-specific mean length at 50% maturity (L50) was 3...

Evaluating Management Regimes using Per-Recruit Models and Relative Stock Density for Mississippi’s Spotted Seatrout

AbstractThe Spotted Seatrout Cynoscion nebulosus is the most popular target of recreational inshore fisheries in Mississippi coastal waters. The Gulf of Mexico (GOM) stock of Spotted Seatrout is composed of spatially distinct substocks, and each state imposes unique bag and size limits. In Mississippi, the stock is managed using minimum length limits and daily bag limits. We used two methods to evaluate the efficacy of length restrictions and fishing mortality (F) levels: (1) a per-recruit model simulation to evaluate the effects of proposed management actions on reproductive output and yield, and (2) an evaluation of how management regimes impact relative stock density (RSD). Relative stock density has been widely used as a management tool in recreational and generally freshwater fisheries but has not been widely employed in informing management of marine stocks. We used demographic information from fisheries-independent sampling and length-specific natural mortality estimates to construct both models. O...

Environmentally driven fluctuations in condition factor of adult Gulf menhaden (Brevoortia patronus) in the northern Gulf of Mexico

We evaluated the effects of a suite of environmental predictors on the relative condition (the relationship between individual weight and length) of adult Gulf Menhaden Brevoortia patronus sampled from the commercial fishery, from 1964 to 2011. A hierarchical linear model (HLM) was used to examine the association between relative condition and Mississippi River discharge. Generalized Additive Models (GAMs) were constructed to determine the impact on relative condition by El Niño Southern Oscillation (ENSO) and the spatially-variant influence of sea surface temperature (SST°C), chlorophyll a concentration (mg m) and wind vector components. The HLM revealed a positive correlation between Mississippi River discharge and relative condition that was consistent throughout the fishing season (April to October). Comparisons of spatially–variant and –invariant GAMs indicated that the effects of SST was consistent in the northern Gulf of Mexico (NGOM) and that the greatest relative condition was at temperatures associated with the Mississippi River plume front. The effect of wind vector components was spatially-variable, likely due to geographic differences in wind-related transport of productive plume waters across the NGOM. Relative condition was greatest during positive ENSO anomalies. Relative condition exhibited intra-annual variability with a small peak during April and May and increasing condition from August until November, likely caused by increased food availability and provisioning by individuals for spawning. We show that multiple bottom-up processes impact the individual dynamics of Gulf Menhaden in the NGOM and these results can be used to predict their impacts on the fisheries and ecology of the NGOM.

Growth, mortality, and movement of cobia (Rachycentron canadum)

1 Division of Coastal Sciences The University of Southern Mississippi 703 East Beach Drive Ocean Springs, Mississippi 39564 Present address for contact author: Department of Evolution, Ecology, and Organismal Biology The Ohio State University 1314 Kinnear Road Columbus, Ohio 43212 2 Center for Fisheries Research and Development Gulf Coast Research Laboratory The University of Southern Mississippi 703 East Beach Drive Ocean Springs, Mississippi 39564