Modeling effects of increased larval mortality on bay anchovy population dynamics in the mesohaline Chesapeake Bay:evidence for compensatory reserve

Abstract

We applied an individual-based population model to examine the potential compensatory scope of the bay anchovy Anchoa mitchilli in Chesapeake Bay. Model simulations were analyzed to estimate: (1) how losses of individuals in different life stages affect year-class production; (2) the changes needed in individual processes and, simultaneously in multiple processes, to offset a 50% increase in larval-stage mortality; and (3) population responses to increasing larval-stage mortality under conditions of presumed high compensatory potential. We hypothesize that, in response to lower densities, the bay anchovy population could compensate for increased larval n~ortality through increased growth rates, increased allocation of energy to reproduction, or reduced predat~on mortality as predators target other species. Simulation results indicate that late-larval and juvenile bay anchovy may be able to consume a significant fraction of their zooplankton prey, suggesting that anchovy is at abundances that can cause density-dependent growth in the Chesapeake Bay. However, densitydependent effects on prey resources alone had a limited buffering effect against a 50% reduction in larval-stage survival. The potential effect of losses of larvae on future production of a year class depended upon when during the larval stage individuals are removed from the population. Modeled alone, large changes in spawning ~ntensity (no. of batches and eggs per batch), egg survival, or mortality of juveniles and adults were required to offset increased larval mortality. When all processes were varied simultaneously, much smaller changes were required. Under a high compensation scenario, there was a strong dome-shaped response in adult production potential to increased larval mortality, such that highest adult production occurred when survival rate of larvae was reduced by as much as 60% While the information presently available to examine density-dependent population responses in bay anchovy is limiting, the modeled results indicate that the bay anchovy population in Chesapeake Ray potentially can regulate its abundance through simultaneous shifts in processes believed to be sensitive to population density.

An individual-based model of bay anchovy population dynamics in the mesohaline region of Chesapeake Bay

“…Cowan et al (1999) and Wang et al (1997) circumvented representing spawning and mortality rates as functions of anchovy density and used the 1-box model to investigate aspects of bay anchovy population dynamics and production. Cowan et al (1999) demonstrated that: (1) density-dependent effects on prey alone had a limiting buffering effect against a 50% reduction in larval-stage survival, (2) the effects of losses of larvae on future production were orders of magnitude higher for larvae and juveniles compared to eggs and yolk-sac larvae, and (3) realistic, simultaneous changes in spawning intensity and post-larval mortality rates can offset a 50% decrease in larvalstage survival. Wang et al (1997) used the anchovy model to show how delayed and bimodal spawning peaks affected long-term population dynamics that are moderated by density-dependent survival and growth.…”

Compensatory density dependence in fish populations: importance, controversy, understanding and prognosis

“…Cowan et al . (1999) used the model to examine the maximum compensatory response of bay anchovy, measured in terms of the largest amount of additional larval mortality that could be offset by compensatory increases in survival or reproduction in other life stages.…”

“…The maximum compensatory response of the population was then simulated by assuming that fecundity, egg mortality, and juvenile and adult mortality would all respond to lowered anchovy densities. The magnitude of the changes in each of these parameters was derived from an extensive review of the literature on bay anchovy population dynamics (Cowan et al . 1999).…”

Size-dependent vulnerability of juvenile bay anchovy Anchoa mitchilli to bluefish predation: Does large body size always provide a refuge?

“…The life history strategy of bay anchovy has been described as 'opportunistic', because they demonstrate a high intrinsic rate of population growth due to early maturation, batch spawning, fast growth rates of larvae, and rapid population replacement . Cowan et al (1999) conclude that this life history strategy is well suited to ensure cohort persistence in the face of heavy mortality based upon bay anchovy achieving growth to mortality ratios greater than 1 (G:Z > 1) at small body sizes. Their modeling results demonstrate that cohorts of bay anchovy begin to increase biomass at about 8 to 10 mm TL with peaks in G:Z occurring in late larval to early juvenile stages (25 to 40 mm TL) .…”

“…Generally, mortality rates for juveniles are estimated to remain high based on annual mortality rates for entire cohorts which can be as high as 95% yr -1 (Newberger & Houde 1995). However, because of a lack of empirical evidence for a size-based mortality relationship in older life stages, Cowan et al (1999) used fixed size-independent rates of mortality for juveniles and adults in a bay anchovy population model in Chesapeake Bay.…”