Allee Effects, Evolution, and the Recovery of Marine Fishes: Reconsideration and Reevaluation
Friday seminar by Jeffrey Hutchings
The recovery of depleted marine fishes has been of increasing interest over the past 20 years. Among the hypothesized correlates of recovery, a key one pertains to population dynamics at low abundance. Classical models, applied widely in fisheries science, assume that per capita population growth rate (r) increases as abundance declines (compensation). In other words, as populations decrease, offspring production per parent is assumed to continually increase with the highest per capita growth rate (rmax, a proxy for individual fitness) occurring at lowest abundance.
This assumption is one of several that have had, and continue to have, great influence on the perceptions of many (most?) fisheries scientists (and increasingly fisheries managers and politicians) of the recovery potential of marine fishes. Some rather prevalent assumptions include the following:
- Marine fishes have higher rates of per capita growth (rmax) than other species;
- Marine fishes have higher rmax because of extraordinarily high fecundity;
- Marine fish populations do not express Allee effects (i.e., depensation);
- Magnitude of population reduction has no effect on the recovery of marine fishes;
- Fisheries-induced evolution is highly problematic and something to be avoided.
Given the slow or absent recovery currently observed for many marine fish populations, despite massive reductions in fishing mortality, it is an opportune time to revisit and reconsider these assumptions.
Here, I present the results of several recent meta-analyses, the first of which shows that rmax is no greater for marine fish than it is for other vertebrates and that fecundity is not related to recovery potential (as argued, in effect, since the late 1800s). Contrary to work undertaken in the 1990s, a second meta-analysis finds evidence an Allee effect in some species, such as Atlantic cod. (An Allee effect is said to exist when per capita population growth rate (r) declines, rather than increases, below a threshold low level of abundance.) This raises questions such as: (i) At what level of abundance (relative to carrying capacity) are Allee effects likely to be manifested? (ii) What are the consequences of Allee effects for the mean and variance in recovery times? A third meta-analysis suggests an intriguing and unexpectedly positive role for fisheries-induced evolution and population recovery.
Dept Biology, Dalhousie University, Halifax, CANADA
CEES, University of Oslo, NORWAY