Ransom A. Myers

WHY DO FISH STOCKS COLLAPSE? THE EXAMPLE OF COD IN ATLANTIC CANADA

In 1993, six Canadian populations of Atlantic cod (Gadus morhua) had collapsed to the point where a moratorium was declared on fishing. It has been argued that the collapses were caused by poor recruitment of cod to the fishery. Yet we are unable to detect a difference between the recruitment of year classes that should have contributed most to the spawning stock at the time of the collapse and recruitment levels in earlier years. A power analysis shows that we would have almost certainly detected an overall reduction of recruitment of 20%. There are considerable differences in the abundance trends as determined by research surveys and reconstructed from the commercial catch at age data (called “virtual population analysis” [VPA]) for each stock. VPA-based abundances consistently depict lower recruitment levels than do survey-based estimates in recent years. More important is the observation that from the early 1980s the VPA-based trend shows a decline where none is apparent in the survey-based trend. One...

Population dynamics of exploited fish stocks at low population levels.

Models of population dynamics in which per capita reproductive success declines at low population levels (variously known as depensation, the Allee effect, and inverse density-dependence) predict that populations can have multiple equilibria and may suddenly shift from one equilibrium to another. If such depensatory dynamics exist, reduced mortality may be insufficient to allow recovery of a population after abundance has been severely reduced by harvesting. Estimates of spawner abundance and number of surviving progeny for 128 fish stocks indicated only 3 stocks with significant depensation. Estimates of the statistical power of the tests strengthen the conclusion that depensatory dynamics are not apparent for fish populations at the levels studied.

The evolution of alternative mating strategies in variable environments

SummaryWe assessed the influence of phenotypic plasticity in age at maturity on the maintenance of alternative mating strategies in male Atlantic salmon,Salmo salar. We calculated the fitness,r, associated with the parr and the anadromous strategies, using age-specific survival data from the field and strategy-specific fertilization data from the laboratory. The fitness of each strategy depended largely on mate competition (numbers of parr per female, i.e. parr frequency) and on age at maturity. Fitness declined with increasing numbers of parr per female with equilibrium frequencies (at which the fitnesses of each strategy are equal) being within the range observed in the wild. Equilibrium parr frequencies declined with decreasing growth rate and increasing age at maturity. Within populations, the existence of multiple age-specific sets of fitness functions suggests that the fitnesses of alternative strategies are best represented as multidimensional surfaces. The points of intersection of these surfaces, whose boundaries encompass natural variation in age at maturity and mate competition, define an evolutionarily stable continuum (ESC) of strategy frequencies along which the fitnesses associated with each strategy are equal. We propose a simple model that incorporates polygenic thresholds of a largely environmentally-controlled trait (age at maturity) to provide a mechanism by which an ESC can be maintained within a population. An indirect test provides support for the prediction that growth-rate thresholds for parr maturation exist and are maintained by stabilizing selection. Evolutionarily stable continua, maintained by negative frequency-dependent selection on threshold traits, provide a theoretical basis for understanding how alternative life histories can evolve in variable environments.

THE LIMITS OF EXPLOITATION: A PRECAUTIONARY APPROACH

As is well known, the biological limit to the exploitation rate of a fish stock is determined by the maximum per capita reproductive rate and the age selectivity of the fishery. Customarily, the selectivity is treated as fixed when the biological limit to harvesting is estimated. From a precautionary perspective, one should control selectivity. A simple model is used to demonstrate the safety benefits of prohibiting the harvesting of juvenile fish. The model leads to a natural rescaling of the maximum per capita recruitment rate, and it is shown that if this quantity is >1, then a spawn-at-least-once policy will prevent a collapse of the stock if fishing mortality targets are exceeded. (A spawn-at-least-once policy requires that fish become vulnerable to commercial gear only after having spawned once.)

The influence of Hudson Bay runoff and ice‐melt on the salinity of the inner Newfoundland Shelf

Abstract The present study examines sources of the interannual variability in salinity on the Newfoundland continental shelf observed in a 40‐year time series from an oceanographic station known as Station 27. Specifically, we investigate, through lag‐correlation analysis, the a priori hypotheses that the salinity anomalies at Station 27 are determined by freshwater runoff anomalies from Hudson and Ungava bays and by ice‐melt anomalies in Hudson Bay and on the Labrador Shelf. Interannual variations of summer runoff into Hudson Bay were significantly negatively correlated with salinity anomalies on the Newfoundland Shelf with a lag (9 months) that is consistent with expected travel times based on known current velocities in Hudson Bay and along the Labrador Shelf. Sea‐ice extent over the Labrador and northern Newfoundland shelves was significantly negatively correlated with salinity at a lag of 3 to 4 months, corresponding to the time of minimum salinity at Station 27. It appears that ice‐melt over the Lab...

Selection against parr maturation in Atlantic salmon

Abstract The feasibility of using scale characteristics to detect maturation as parr of anadromous male Atlantic salmon was examined. Smolt age and growth rate were found to be reliable indicators of prior maturation, whereas scale morphology did not differ between maturation phenotypes. Although determination of prior maturation was not 100% efficient, the resulting selection intensities would allow for effective artificial selection against anadromous males which had matured as parr.

A spurious correlation in an interpopulation comparison of Atlantic Salmon life histories

We tested two hypotheses concerning geographical variation in Atlantic salmon (Salmo salar) life histories: (1) mean age at first reproduction is positively correlated with growth rate at sea and (2) within-population variation in age at first reproduction first increases and then decreases with latitude. Data on growth and age at first reproduction were compiled from 41 populations in eastern North America. Data reliability was checked by a redetermination of ages based on scale examination. The proportion of fish that were incorrectly aged was small (°0.7%); however, aging errors were primarily of one kind; salmon that had previously spawned were misclassified as virgin fish of an older age class. Growth rate at sea was found not to be positively correlated with age at maturation. Schaffer and Elsons (1975) positive correlation between growth and age at first reproduction can be attributed to a subtle statistical artifact caused by aging errors. We also found that within-population variation of age at maturation was not related to latitude. We conclude that tests of life history theories should not assume constancy in life history traits, such as mortality, among populations.