Population dynamics and potential of fisheries stock enhancement: practical theory for assessment and policy analysis

The population dynamics of fisheries stock enhancement, and its potential for generating benefits over and above those obtainable from optimal exploitation of wild stocks alone are poorly understood and highly controversial. I review pertinent knowledge of fish population biology, and extend the dynamic pool theory of fishing to stock enhancement by unpacking recruitment, incorporating regulation in the recruited stock, and accounting for biological differences between wild and hatchery fish. I then analyse the dynamics of stock enhancement and its potential role in fisheries management, using the candidate stock of North Sea sole as an example and considering economic as well as biological criteria. Enhancement through release of recruits or advanced juveniles is predicted to increase total yield and stock abundance, but reduce abundance of the naturally recruited stock component through compensatory responses or overfishing. Economic feasibility of enhancement is subject to strong constraints, including trade-offs between the costs of fishing and hatchery releases. Costs of hatchery fish strongly influence optimal policy, which may range from no enhancement at high cost to high levels of stocking and fishing effort at low cost. Release of genetically maladapted fish reduces the effectiveness of enhancement, and is most detrimental overall if fitness of hatchery fish is only moderately compromised. As a temporary measure for the rebuilding of depleted stocks, enhancement cannot substitute for effort limitation, and is advantageous as an auxiliary measure only if the population has been reduced to a very low proportion of its unexploited biomass. Quantitative analysis of population dynamics is central to the responsible use of stock enhancement in fisheries management, and the necessary tools are available.     

Coral reef fisheries stock assessment

Summary Although all fisheries are multispecies and spatially heterogeneous, coral reef fisheries are an extreme in both respects. The two main approaches to stock assessment have been either to consider individual species separately or to lump all species together. Both are limited in their predictive power. The lack of ecological knowledge and the large number of parameters required make methods based on single species often impractical or expensive. However, where the appropriate information is available, ecological studies do not provide significant improvements on yield-per-recruit or surplus yield models currently used.Alternative community models based on aggregates of species lack predictive power, empirical support and relative data. Models are further limited because they do not address various economic aspects, fish movement and recruitment, all of which must be spatially resolved. However, the ECOPATH model, based on trophic compartments, represents a new approach useful to multispecies assessment, and the only way at present to include predation in stock models. In practice the data available will be the most important factor in the choice of stock assessment model.The major concern for management of many coral reefs is conservation of the habitat and stocks. In some cases this has been achieved with little reference to stock assessment, by using community management and closed areas, which are receiving increasing support. However, once the objective of conservation is achieved, stock assessment should have an important role in improving the economic performance of the fisheries. The wider problems of management have no simple solutions, but managers should look to adaptive management, designing their own experiments to choose between management models.     

Salmon stock and recruitment, and stock enhancement

The precise nature of the stock and recruitment relationship in natural populations is of vital importance to any attempt at stock enhancement by removing or by-passing a limiting factor, or at management by manipulation of spawning stock. In this paper the evidence for and against a dome-shaped stock and recruitment curve is reviewed, and consideration given to several factors which complicate the conventional, simplistic interpretation of density-dependent mortality. These include environmentally mediated changes in 'carrying capacity', distribution of spawning, and the need to consider whole river systems. The implications of these factors for fisheries management and stock enhancement are discussed.     

Stock enhancement as a fisheries management tool

Reviews in Fish Biology and Fisheries - Stock enhancement has been viewed as a positive fisheries management tool for over 100 years. However, decisions to undertake such activities in the past...     

Stock enhancement as a fisheries management tool

Stock enhancement has been viewed as a positive fisheries management tool for over 100 years. However, decisions to undertake such activities in the past have often been technology-based, i.e., driven by the ability to produce fishes, with most stock enhancement projects having limited or no demonstrated success. The reasons for this have been due to an inability to identify and/or control the underlying reasons why a fishery is under-performing or not meeting management objectives. Further, stock enhancement has often been applied in isolation from other fisheries management tools (e.g., effort control). To address these issues and consider stock enhancement in a broader ecosystem perspective, a new approach for stock enhancement is proposed. The proposed model comprises four major steps; a review of all information about an ecosystem/fishery/stock and the setting of clear management targets; a comparison of all relevant fisheries management tools with the potential to meet the management targets; the instigation of a scientifically based, pilot-scale, stock enhancement program with clear objectives, targets, and evaluations; and a full-scale stock enhancement program if the pilot project meets the objectives. The model uses a flow-chart that highlights a broad range of scientific and other information, and the decisions that need to be made in relation to stock enhancement and fisheries management in general. In this way all steps are transparent and all stakeholders (managers, scientists, extractive and non-extractive users, and the general public) can contribute to the information collection and decision making processes. If stock enhancement is subsequently identified as the most-appropriate tool, then the stepwise progression will provide the best possible chance of a positive outcome for a stock enhancement project, while minimizing risks and costs. In this way, stock enhancement may advance as a science and develop as a useful fisheries management tool in appropriate situations.     

Population biology and the management of fisheries

The successful management and exploitation of fisheries requires detailed knowledge of the population dynamics and life histories of commercial species, and of the factors affecting recruitment and mortality. Much progress has been made in recent years, exemplified by the report of the 1984 Dahlem Workshop on the exploitation of marine communities, and by subsequent research discussed in this article. In some fisheries at least, management decisions can now have a firm scientific base.     

Misidentification of istiophorid billfishes by fisheries observers raises uncertainty over stock status

We investigated whether commercially landed black marlin Istiompax indica were being misidentified by fisheries observers operating throughout the Pacific Ocean. Of 83 samples reported by observers as I. indica, 77% were genetically identified to be blue marlin Makaira nigricans and 2% to be striped marlin Kajikia audax. The high rate of misidentification by observers places considerable uncertainty over historic catch ratios of Indo‐Pacific marlin and stock assessments relying on the validity of these data.     

Status of research in stock enhancement and sea ranching

Reviews in Fish Biology and Fisheries -     

Population dynamics of stored-product mites

Summary Population fluctuation patterns were determined during 1959–70 in wheat bulks in 2 granaries typical of those used on Manitoba farms for 7 kinds of mites. The patterns were bradly similar in the 2 granaries. Outbreaks of all mites except Tarsonemus granarius and Aëroglyphus robustus were unaffected by the prolonged storage of grain; those of Acarus siro and Cheyletus eruditus occurred in unusually high numbers periodically every 2–5 years, whereas Glycyphagus destructor and Androlaelaps casalis were present in relatively low numbers from the first year of storage. A rise in T. granarius population corresponded with grain age and the succession of microfloral species. Tydeid mites occurred periodically.Pricipal component analyses of up to 26 variates including various fungal species on one grain bulk ecosystem provided a kaleidoscopic view of interrelations and dynamics of acarine populations in the grain bulks. The combination of correlated variates of which mites are a part, varied from year to year.Four factors that regulate acarine numbers, particularly those of A. siro and its natural predator C. eruditus, were identified. These are: (i) basic determinants (moisture, temperature, food, and the intrinsic rate of increase of the species); (ii) the main regulator (temperature); (iii) influencers (seed cracks, microflora, dockage, etc.); and (iv) seasonal and cyclic regulators (seasonality in temperature and several density-dependent factors). Generalized diagrams of prey-predator interactions and multivariate interrelations of each kind of mite are presented.Contribution no. 568 from Research Station, Agriculture Canada, Winnipeg, Canada. Invitation paper presented by the first author at the Joint Annual Meeting of the Entomological Societies of America, Canada, and Quebec, Montreal, Quebec, on 26 November 1972     

Population dynamics of gene transfer

Horizontal transfer of nucleic acid sequences is a widespread process of genetic change in bacterial populations, one taking on a variety of specific forms. A model capturing some of the general features of this process is formulated. Two major subsidiary variants of this model are analyzed. The first lacks competition between cells. In this case, the transferred genetic element can approach fixation asymptotically if cells bearing it are viable, but not otherwise. The second variant of the proposed model has competition. This gives rise to complex dynamical possibilities, but never fixation of the transferred genetic element. It is argued that the evolutionary transition from gene transfer polymorphism to obligatory genetic exchange, and thus the origin of sex, requires either population dynamics which are not subject to growth rate density-dependence or cell-dependence on transferred genetic elements, although such dependence does not require that the element be beneficial.     

Population dynamics of obligate cooperators

Obligate cooperative breeding species demonstrate a high rate of group extinction, which may be due to the existence of a critical number of helpers below which the group cannot subsist. Through a simple model, we study the population dynamics of obligate cooperative breeding species, taking into account the existence of a lower threshold below which the instantaneous growth rate becomes negative. The model successively incorporates (i) a distinction between species that need helpers for reproduction, survival or both, (ii) the existence of a migration rate accounting for dispersal, and (iii) stochastic mortality to simulate the effects of random catastrophic events. Our results suggest that the need for a minimum number of helpers increases the risk of extinction for obligate cooperative breeding species. The constraint imposed by this threshold is higher when helpers are needed for reproduction only or for both reproduction and survival. By driving them below this lower threshold, stochastic mortality of lower amplitude and/or lower frequency than for non-cooperative breeders may be sufficient to cause the extinction of obligate cooperative breeding groups. Migration may have a buffering effect only for groups where immigration is higher than emigration; otherwise (when immigrants from nearby groups are not available) it lowers the difference between actual group size and critical threshold, thereby constituting a higher constraint.     

Population dynamics of flocculating yeasts

Abstract: The problem of understanding the recognition and specific interactions in a population of yeast flocculating cells is discussed. The biochemistry, physiology and genetics of flocculation is briefly reviewed. Yeast flocculation requires the expression of a specific protein (lectin) on flocculent cells, and carbohydrate (receptors) on neighbouring cells. Adhesion experiments performed with cells whose flocculation is repressed by growth conditions, indicating that the inhibition of flocculation is due to inhibition or inactivation of 'lectin-like' component. Additionally, using adhesion experiments, it is demonstrated that cells of non-flocculent strain interact by establishing a true bond with flocculent cells rather than by entrapment inside the floc matrix. As phenotypic expression of flocculation, for several strains, is shown to be repressed, modulated or induced by modifying growth conditions, the constitutiveness and inducibility of flocculation are also discussed.     

Population dynamics of gyrinid beetles

Summary Survival chances have been estimated for populations of the waterbeetle Gyrinus marinus Gyll. from capture-recapture sequences in 1974–1978. The average weekly survival chance is estimated as 0.9247 in spring, decreasing to 0.7701 in autumn; males survive longer than females. Survival shows little variation in time or space (variation coefficient V<0.1). Survival chance did not significantly decrease with age. Freshly emerged tenerals apparently have a lower survival chance than older beetles. The survival chance during hibernation (October–April) is about 0.33, with a greater variation between years (V=0.7) than between populations (V=0.4).Communication N⁰ 301 of the Biological Station, Wijster