Victor R. Restrepo

High Value and Long Life—Double Jeopardy for Tunas and Billfishes

The first standardized, global assessment of these fishes, using Red List criteria, reveals threatened species needing protection. There is growing concern that in spite of the healthy status of several epipelagic (living near the surface) fish stocks (1), some scombrid (tunas, bonitos, mackerels, and Spanish mackerels) and billfish (swordfish and marlins) species are heavily overfished and that there is a lack of resolve to protect against overexploitation driven by high prices (2–5). Many populations are exploited by multinational fisheries whose regulation, from a political perspective, is exceedingly difficult. Thus, assessment and management is complicated and sometimes ineffective (4). Regional Fisheries Management Organizations (RFMOs) were created to manage and conserve scombrids and billfishes because of their transnational distributions and widespread economic importance (6). However, species-specific catch data for many scombrids and billfishes are not collected or are aggregated with other species. Even for the larger tunas, for which relatively rich data exist, population assessments and data are complex (1) and are difficult to combine across RFMOs, which prompts a need for alternative means of assessment.

Environmental influences on potential recruitment of pink shrimp, Farfantepenaeus duorarum, from Florida Bay nursery grounds

Two modeling approaches were used to explore the basis for variation in recruitment of pink shrimp,Farfantepenaeus duorarum, to the Tortugas fishing grounds. Emphasis was on development and juvenile densities on the nursery grounds. An exploratory simulation modeling exercise demonstrated large year-to-year variations in recruitment contributions to the Tortugas pink shrimp fishery may occur on some nursery grounds, and production may differ considerably among nursery grounds within the same year, simply on the basis of differences in temperature and salinity. We used a growth and survival model to simulate cumulative harvests from a July-centered cohort of early-settlementstage postlarvae from two parts of Florida Bay (western Florida Bay and northcentral Florida Bay), using historic temperature and salinity data from these areas. Very large year-to-year differences in simulated cumulative harvests were found for recruits from Whipray Basin. Year-to-year differences in simulated harvests of recruits from Johnson Key Basin were much smaller. In a complementary activity, generalized linear and additive models and intermittent, historic density records were used to develop an uninterrupted multi-year time series of monthly density estimates for juvenile pink shrimp in the Johnson Key Basin. The developed data series was based on relationships of density with environmental variables. The strongest relationship was with sea-surface temperature. Three other environmental variables (rainfall, water level at Everglades National Park Well P35, and mean wind speed) also contributed significantly to explaining variation in juvenile densities. Results of the simulation model and two of the three statistical models yielded similar interannual patterns for Johnson Key Basin. While it is not possible to say that one result validates the other, the concordance of the annual patterns from the two models is supportive of both approaches.

Closed Seasons and Tropical Penaeid Fisheries: A Simulation Including Fleet Dynamics and Uncertainty

Abstract Seasonal fishery closures are commonly used in fisheries management for various purposes, including limitation of effort, protection of spawners, and maximization of the yield or value that can be obtained from a cohort. The effectiveness of a proposed closure can be evaluated through yield-per-recruit analysis, which can be carried out analytically for some simple situations. For other fisheries, such as the penaeid shrimp fishery of Torres Strait, Australia, investigated here, the analyses are more complex because recruitment occurs in pulses throughout the year and the intensity of fishing is itself unevenly distributed in time, being patterned after these recruitment pulses. Furthermore, the imposition of closures of different durations has been documented to alter the pattern and intensity of fishing after the fishery reopens. In this study, a simulation approach is used to identify the timing and duration of closures that are likely to increase the yield or the value per recruit of the fish...

Equilibrium Production Models that Incorporate Fished Area

Abstract Users of stock production models assume that a time series of catch-per-effort data reflects changes in the size of a self-sustaining biological population. This assumption is often violated in analyses of fisheries that have undergone changes in the areal coverage of the fishing fleet because the fraction of the population that is available to fishing also changes. Furthermore, the problem is aggravated when there is biomass transfer (in the form of migration) between the exploited and unexploited segments of the population. We present a general production model that can be used to understand the dynamics of equilibrium population size and yield as functions of fishing mortality and exploitable population fraction. We then develop two simple subsets of this model in which biomass transfer is either nil or high between the unexploited and exploited fractions, and we discuss methods to estimate the models parameters given data on catch, effort, and area. The application of the models is illustrat...

Growth estimates for male stone crabs along the southwest coast of Florida: a synthesis of available data and methods

Abstract Little is known about the growth of male stone crabs Menippe mercenaria in Florida waters. For the most part, information from laboratory and tagging data has not been analyzed thoroughly to obtain a growth curve for the species. In this paper, I synthesized existing and new methods to construct a growth curve from available data. Two components of growth were estimated in detail: the relationship between postmolt and premolt size and the relationship between intermolt period and premolt size. The first relationship was estimated from data from captive crabs by relating the sizes before and after ecdysis with a segmented regression model. The relationship between intermolt period and premolt size was estimated from data from laboratory studies in which crabs were allowed to molt once and the subsequent intermolt periods were recorded exactly. Because long exposure to laboratory conditions may affect growth rates, the molt–frequency relationship also was estimated with methods that require one of ...

Utility-per-Recruit Modeling: A Neglected Concept

Abstract Yield-per-recruit analysis consists of summing or integrating the relative weight of the catch over all ages exploited in a fishery. Utility-per-recruit analysis is a generalization of this in which the utility or value (not necessarily monetary) of the catch, instead of the weight, is accumulated over age, The utility function of age can take many forms. For a sport fishery, age-specific ratings offish quality can be derived from angler interviews in order to construct the utility function. In commercial fisheries, utility would most generally be taken to be the net income per recruit (revenues minus costs). However, it will often be of interest to examine per-recruit revenue alone because this is easy to obtain. In several important cases, the computations can be performed analytically.

Parameter Uncertainty and Simple Yield-per-Recruit Analysis

Abstract Simple yield-per-recruit analysis is a useful stock assessment tool for fishery management. We present a Monte Carlo-based method that incorporates available knowledge of parameter uncertainty on the three-parameter yield-per-recruit model of Beverton and Holt. The uncertainty associated with planning specific changes in yield per recruit due to regulation can thus be examined, conditional on the uncertainty in the models inputs. The application of the method is illustrated with available information on the Florida fishery for red drum Sciaenops ocellatus. The results clearly show that this method is superior to traditional types of analyses in which only the midpoint and extreme input parameter estimates are used.

Estimating the Intermolt Periods in Asynchronously Molting Crustacean Populations

SUMMARY In some crustacean populations, including many tropical lobsters and crabs, molting occurs asynchronously throughout the year. That is, at any time of year, the animals are uniformly distributed throughout the intermolt cycle. We present methods for finding maximum likelihood estimates of the size-specific intermolt period for this situation when mark-recapture data are available consisting of initial size, time at liberty, and whether the animal molted while at liberty. Alternatively, the intermolt period can be determined by the same estimation procedure when wild animals are brought into captivity and observations are made on which animals molt within some fixed time period or before a fixed date. The method is applied to data on spiny lobster (Panulirus argus) to estimate intermolt period as an exponential function of length. Crustacean growth is a discontinuous process that is often described in terms of two components: the change in size at molt and the duration of the intermolt period (Botsford, 1985). Information on these processes is used to construct a model of growth in size over time for use in fishery assessment models. Both growth components vary with the size of the animal. These can be studied by observing animals held in captivity though the results may not be representative of growth in wild populations. An alternative is to estimate the components of growth from mark-recapture data. In this paper, we focus on the problem of estimating the intermolt period under the assumption that the individuals in a population at any time are uniformly distributed throughout their intermolt period. We use the method of maximum likelihood to derive a procedure for estimating the intermolt periods of tropical and subtropical decapods from mark-recapture data when seasonal changes in growth are negligible. The procedures can also be used to analyze laboratory data on the proportion of animals molting when animals are held for periods of time sufficiently short to minimize the effects of captivity. In Section 2, we consider mark-recapture studies and develop the estimators for intermolt period as a function of body size and other covariates. The growth of spiny lobsters