Major Research Topics

Temperature-dependent sex determination and sex ratio evolution

A long-standing interest is to understand how the sex ratio evolves. I have been the first to show that sex determination in fishes is influenced by temperature during larval development. Most of this work has involved the Atlantic silverside, Menidia menidia, but the phenomenon is probably widespread. These findings are important not only in designing approaches to sex ratio manipulation in aquaculture, but also to understanding the causes of fluctuations in sex ratio among natural populations.

Countergradient variation in growth

This area of investigation concerns how growth rate is adapted to differences in seasonality that occur with latitude. In several species distributed along the east coast of North America, the length of the growing season declines with increasing latitude by a factor of about three. Yet body size at the end of the growing season is independent of latitude. Experimental studies on laboratory-reared fish explain this paradox: high-latitude fish have a higher genetic capacity for growth and grow two to three times faster within the growing season than do low-latitude fish. This "countergradient variation" in growth rate is now known to be widespread in fishes and may provide a general model for choosing natural stocks to be used in aquaculture: natural populations with the highest capacity for growth may be found where the growing season is shortest. In fact, studies we have conducted on Atlantic silversides, striped bass, and the common mummichog prove that northern strains grow much faster than those from the south under intensive aquaculture conditions.

Trade-offs in the evolution of growth rate

The existence of genetic variation in growth also implies that there must be evolutionary trade-offs that select for different growth rates at different latitudes. Much of our current research is directed at establishing the selective pressures that cause genetic variation in growth rate. This much is known so far. First, rapid growth is required at higher latitudes because large size is required to survive the long winters up north (see below). But why wouldn't rapid growth and large size also be beneficial to fitness in southern environments with short winters? We have demonstrated through laboratory studies of performance that there is a cost to rapid growth: fish that grow rapidly, and consume large meals to do so, have lower size-specific swimming ability (both burst and sustained swimming) and they are more susceptible to predators. Hence, rapid growth trades-off with defensive capabilities. Current work involves the development of mathematical models to address the theoretical (evolutionary) implications of this trade-off and field and mesocosm studies to demonstrate the existence of trade-offs in growth rate in nature.

Darwinian fishery science

Although fishery science has embraced the importance of life history variation in predicting the effects of harvest on population ecology, evolutionary change in life history has received relatively little attention. By focusing only on the ecological effects, standard fisheries science implicitly assumes either that genetic influences on physiology and life history in the wild are negligible or that natural selection is a slow process that can be effectively ignored. Disentangling genetic and environmental influences is difficult for most species and the resultant lack of contrary evidence has allowed these assumptions to persist. Drawing upon >25 years of research on the Atlantic silverside Menidia menidia, our research shows that adaptive genetic variation in many traits is finely tuned to natural gradients in climate, much more so than would be predicted from estimates of gene flow. Much of this variation is caused by a gradient in size-selective winter mortality and involves 2-3 fold changes in physiological traits that influence population productivity. Many other species are now known to display similar patterns. Harvest experiments show that these traits can evolve rapidly in response to size-selective fishing. Hence, the pool of genotypes that code for life history traits is a highly dynamic property of populations. We argue that the lessons from Menidia are applicable to many exploited species where similar observations would be difficult to obtain and advocate greater use of species models to address fundamental questions in fishery science. Present research is focused on the reversibility of harvest-induced genetic change and on the suite of life history, physiological, behavioral, and morphological traits that coevolve under size-selective harvest regimes.