Fish compensatory responses following whole-lake experimental density reduction.

摘要

I evaluated fish compensatory responses following a whole-lake experimental density reduction of gizzard shad at Lake Dora, Florida. Gizzard shad were removed in 2005 and 2006 via a subsidized commercial fishery by the St. Johns River Water Management District. Approximately 30% of the total gizzard shad biomass and 70% of the total spawner biomass was removed. My first objective was to develop an age and length-structured population model that could be used to assess recruitment responses following density reduction. The model provided unbiased estimates of mortality, growth, gear selectivity and recruitment for long-lived species. For short-lived species there was a small upward bias in early recruitment estimates when the time series length was half of the maximum age, but all other scenarios provided unbiased estimates. My second objective was to evaluate compensatory responses of gizzard shad growth, maturity, and pre-recruit survival following density reduction. I found no evidence for density dependence in growth and maturity. Pre-recruit survival was negatively related to spawner biomass suggesting density dependence in early life survival. The strength of density dependence in pre-recruit survival was quantified by estimating the maximum lifetime reproductive rate, which was 7.3 (95% confidence interval: 1.9--16.5) at Lake Dora. Thus, pre-recruit survival increases 7.3-fold at very low spawner abundance when compared to the unfished condition at Lake Dora. My third objective was to assess the relative efficacy of different gill net mesh sizes, exploitation rate, and harvest interval (number of years between removals) on total biomass reduction and spawning potential ratio of gizzard shad following removals at hypereutrophic Florida lakes using a simulation model. Specifically, I was interested in whether various combinations of these factors could achieve a 75% reduction in total population biomass and Spawning potential ratio (SPR). These targets were obtained from the literature and represent the reductions needed to achieve changes in phytoplankton biomass and to effectively reduce fish recruitment following biomanipulation. Gizzard shad biomass reduction failed to meet the target of 75% reduction for all mesh sizes and harvest intervals except for the smallest (51-mm) mesh size. Reductions in spawning potential ratio exceeded total biomass reductions but remained above a 75% reduction target except when using the smallest mesh size (51 mm). I conclude that gill net removals are unlikely to result in substantial biomass reductions at Lake Dora unless removals adopt very small mesh sizes, which may not be preferred by commercial fishers.