David J. Die

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.

The impact of global positioning systems and plotters on fishing power in the northern prawn fishery, Australia

The impact of global positioning systems (GPS) and plotter systems on the relative fishing power of the northern prawn fishery fleet on tiger prawns (Penaeus esculentus Haswell, 1879, and P. semisulcatus de Haan, 1850) was investigated from commercial catch data. A generalized linear model was used to account for differences in fishing power between boats and changes in prawn abundance. It was found that boats that used a GPS alone had 4% greater fishing power than boats without a CPS. The addition of a plotter raised the power by 7% over boats without the equipment. For each year between the first to third that a fisher has been working with plotters, there is an additional 2 or 3% increase. It appears that when all boats have a GPS and plotter for at least 3 years, the fishing power of the fleet will increase by 12%. Management controls have reduced the efficiency of each boat and lowered the number of days available to fish, but this may not have been sufficient to counteract the increases. Further limits will be needed to maintain the desired levels of mortality.

Stock-recruitment relationships of the tiger prawns (Penaeus esculentus and Penaeus semisulcatus) in the Australian Northern Prawn Fishery

This paper investigates the stock-recruitment and equilibrium yield dynamics for the two species of tiger prawns (Penaeus esculentus and Penaeus semisulcatus) in Australias most productive prawn fishery: the Northern Prawn Fishery. Commercial trawl logbooks for 1970-93 and research surveys are used to develop population models for these prawns. A population model that incorporates continuous recruitment is developed. Annual spawning stock and recruitment indices are then estimated from the population model. Spawning stock indices represent the abundance of female prawns that are likely to spawn; recruitment indices represent the abundance of all prawns less than a certain size. The relationships between spawning stock and subsequent recruitment (SRR), between recruitment and subsequent spawning stock (RSR), and between recruitment and commercial catch were estimated through maximum-likelihood models that incorporated autoregressive terms. Yield as a function of fishing effort was estimated by constraining to equilibrium the SRR and RSR. The resulting production model was then used to determine maximum sustainable yield (MSY) and its corresponding fishing effort (fMSY). Long-term yield estimates for the two tiger prawn species range between 3700 and 5300 t. The fishing effort at present is close to the level that should produce MSY for both species of tiger prawns. However, current landings, recruitment and spawning stock are below the equilibrium values predicted by the models. This may be because of uncertainty in the spawning stock-recruitment relationships, a change in carrying capacity, biased estimates of fishing effort, unreliable catch statistics, or simplistic assumptions about stock structure. Although our predictions of tiger prawn yields are uncertain, management will soon have to consider new measures to counteract the effects of future increases in fishing effort.

Maturation in the female eastern king prawn Penaeus plebejus from coastal waters of eastern Australia, and considerations for quantifying egg production in penaeid prawns

The trawl fishery for eastern king prawns, Penaeus plebejus, is of major commercial significance on the cast coast of Australia. This paper describes the reproductive biology of female P. plebejus and presents new information relevant to quantifying egg production in penacid prawn populations. The study concluded that the gonosomatic index (GSI) covaried with prawn size and was, therefore, likely to be a poor indicator of reproductive activity for penaeid prawn populations. GSI was also found to be not independent of ovary histological stage, moult stage, insemination status and parasitisation by bopyrid isopods. An histological description of ovarian development and the incidence of each stage of development is provided. This includes a description of the ovulation stage, which has not been previously recorded in naturallyoccurring populations. Fifty percent of female P. plebejus were classed as mature at 42 mm CL. The relationships between GSI, percent mature, and percent inseminated with size suggest that there is a decline in the capacity of large (>60 mm CL) females to produce and fertilise eggs. The declines in both GSI and percent mature is indicative of ovarian senescence in large(old) females, which has not been recorded previously in penaeid prawns. An index which considers the relationships between the percent mature, fecundity and the percent inseminated with size, as well as the influence of mortality on the population, is put forward to determine the relative contribution of different size classes of females to egg production. The index suggests that females >50 mm CL contribute little to egg production, and that the bulk of eggs are produced by 35 to 48 mm CL females.

Sustainable yield indicators from biomass: are there appropriate reference points for use in tropical fisheries?

Various methods have been proposed to calculate maximum sustainable yield, MSY, from biomass estimates, but the classical approach of Gulland, MSY = 0.5 M B0 is still widely used, despite overwhelming theoretical and practical evidence pointing to its limited value, probably due to confusion as to the alternative methods available. A review of alternative methods is presented and their applicability to species with high values of M discussed. An extreme-value strategy is proposed for introducing considerations of risk and precaution into a management system based on precautionary reference points. We also focus on problems of estimating MSY and other reference points, review alternative estimators and discuss some of the ecological and life history criteria that affect how these estimators should be used: as targets or limits for exploitation, as indicators of conformity under sustainable harvesting conditions, or even for ecocertification of responsible or sustainable fisheries.

Population dynamics of Scyllarid lobsters of the genus Thenus spp. on the Queensland (Australia) east coast: I. Assessing the effects of tagging

The effect of different combinations of tagging and release methods was examined on the survival, moulting, growth increment and recapture rates of slipper lobsters Thenus spp. on the Queensland (Australia) east coast using generalised linear modelling methods. Laboratory experiments indicated that while tagging is unlikely to significantly affect the survival rate or growth increments of lobsters, it is likely to lower the incidence of moulting. In the field, recapture rates of tagged lobsters increased markedly by applying an antibiotic/antifungal ointment to the tag wound and by minimising the length of time lobsters were held on board prior to release. Recapture rates for Thenus orientalis declined by about 0.5% for every hour lobsters were held on board prior to release, even though they were kept in aerated seawater. About twice as many male T. orientalis were recaptured compared to females. Reasons for this may be related to size differences between the sexes and how the fishers valued the rewards for different size classes. The size of the T-bar anchor tag affected recapture rates of Thenus indicus; recapture rates of lobsters tagged with small tags were about 33% higher than those with large tags. Two different methods of release were also compared; surface release and bottom cage release. Recapture rates of large lobsters and those that were held on board for several hours prior to release were improved by using the bottom cage method. However, under some conditions, the cage may lower recapture rates relative to the surface release method, possibly because of increased stress and trauma from crowding in the cage. For both species, growth increments of recaptured lobsters increased with the period at liberty and declined with increasing lobster size. Females had significantly larger growth increments than males.

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...

Incorporating commercial fishery information into the design of marine protected areas

Marine protected areas (MPAs) are becoming a favoured management strategy for the conservation of marine biodiversity. We present an analysis of how fishery data from a trawl fishery in northern Australia can contribute to the process of developing a system of candidate MPAs. We investigate ways of using fishing grounds, bioregion and depth information as a means of classifying ecosystems, and then show how fishery data could be used in the process of selecting between candidate areas. The way that the fishing grounds are defined as surrogate ecosystems will determine the extent of the impact on fishery. Our analyses highlight some of the benefits of using fishery data in the process of developing MPAs. Early and continued involvement of the fisheries community will ensure that MPAs have a real chance of achieving their main objective of biodiversity conservation whilst possibly reducing the overexploitation of marine fishery resources.

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.

Selectivity of Gill Nets Used in the Commercial Spanish Mackerel Fishery of Florida

Abstract Selection curves of encircling (run-around) and drifting (stab) gill nets used in the winter fishery for Spanish mackerel Scomberomorus maculatus off southern Florida were estimated by use of cumulative probability distributions of retention girth at length. Selection curves corresponded well with observed size frequencies obtained from each mesh size, Increasing differences between maximum and head girth perimeters as fish grew resulted in selection curves indicating increased selection range and efficiency. Selectivity of Spanish mackerel gill nets will change as the condition of the fish changes with the onset of the spawning season. Apparently, selectivity also changes with twine size.