UW Aquatic & Fishery Sciences Quantitative Seminar
Dr. Jameal Samhouri
Northwest Fisheries Science Center
Food matters: density dependence and offspring provisioning in a coral reef fish
Abstract - Replenishment of marine populations occurs primarily through the entry of juveniles to adult populations following a pelagic larval stage. It is widely held that mortality during the pelagic stage is high, density-independent, and determined in part by the quality of eggs and larvae produced by adults. Consequently, larval abundance and traits of individual larvae should have strong effects on overall population dynamics in marine organisms. Surprisingly, few experiments have tested how localized interactions among breeding adults affect the quantity and characteristics of larvae they produce. Here I show that food limitation and population density both influence reproductive output in a planktivorous, territorial coral reef fish, the bicolor damselfish, Stegastes partitus. I manipulated food supply and adult damselfish density on isolated patch reefs near Lee Stocking Island, Bahamas, and monitored subsequent somatic growth and fecundity (i.e., offspring number), along with offspring provisioning (larval length and energy stores). I also monitored fecundity in two natural populations at nearby continuous reefs. Crowding led to slower growth and depressed per capita reproductive output, while augmenting food availability enhanced somatic growth and larval energy stores. Experimental food supplementation did not influence the number of offspring produced per female, however, and neither experimental treatment affected larval length. I used this information and published data describing density-dependent mortality in bicolor damselfish to parameterize a two-patch metapopulation model. Numerical simulations demonstrate that high food and larval supply both produce increased density dependence in mortality but reduced density dependence in growth and fecundity, and vice versa. As a result, introducing spatial heterogeneity in food and larval supply to the model created differences among the patches in population abundance and in larval output. The potential for such spatial differences is an important consideration in the design of marine reserve networks.