FORECASTING FUTURES - THE EFFECT OF AGE, ABUNDANCE, AND HARVESTING ON THE LONG-TERM SUSTAINABILITY OF MARINE FISHES

Abstract

In the first part of this thesis I attempt to address some of the concerns regarding the stock-recruitment relationship that have been voiced over the last 50 years. In chapter 2 I revisit the shape of the stock-recruitment relationship at low abundance. I show that at these low abundances an increase in productivity is not ubiquitous. In many populations the dynamics are essentially density independent after the populations decline below 40% of maximum historic SSB, and in some species the productivity actually starts to decline below this threshold. Given the weakening of compensation in many populations, in chapter 3 I examine how per capita harvest mortality changes with abundance. The results show that in the majority of populations, per capita mortality continues to increase with declines in abundance. In Chapter 4, I attempt to address concerns that have been raised about the effect of age structure on recruitment. Here, I break down the relationship to determine whether there is an effect of first time (virgin) or repeat spawners in terms of average size, and the relative abundance of large and small fish. The results suggest that these age-specific components of the spawning stock contribute differentially to recruitment across a wide range of species and populations. In Chapters 5 and 6 I change the focus and use a technique widely used in terrestrial ecology to estimate both the risk of collapse and the probability of persistence for numerous populations of Atlantic cod (Gadus morhua) and Atlantic herring (Clupea harengus). In chapter 5 the models indicate that periods of unusually elevated recruitment are vital to the persistence of all the populations analyzed. In Chapter 6, I look specifically at the potential impact that different management actions and environmental variability may have on population recovery for an endangered Atlantic cod population in the Gulf of St. Lawrence. Throughout this thesis, I attempt to address problems in fisheries science from a more ecological perspective than that traditionally used in fisheries science. More collaboration between ecologist scientists and fisheries scientist will only help to improve our understanding of population dynamics in both marine and terrestrial ecosystems.