Predator-induced demographic shifts in coral reef fish assemblages

Abstract

In recent years, it has become apparent that human impacts have altered community structure in coastal and marine ecosystems worldwide. Of these, fishing is one of the most pervasive, and a growing body of work suggests that fishing can have strong effects on the ecology of target species, especially top predators. However, the effects of removing top predators on lower trophic groups of prey fishes are less clear, particularly in highly diverse and trophically complex coral reef ecosystems. We examined patterns of abundance, size structure, and age-based demography through surveys and collection-based studies of five fish species from a variety of trophic levels at Kiritimati and Palmyra, two nearby atolls in the Northern Line Islands. These islands have similar biogeography and oceanography, and yet Kiritimati has ∼10,000 people with extensive local fishing while Palmyra is a US National Wildlife Refuge with no permanent human population, no fishing, and an intact predator fauna. Surveys indicated that top predators were relatively larger and more abundant at unfished Palmyra, while prey functional groups were relatively smaller but showed no clear trends in abundance as would be expected from classic trophic cascades. Through detailed analyses of focal species, we found that size and longevity of a top predator were lower at fished Kiritimati than at unfished Palmyra. Demographic patterns also shifted dramatically for 4 of 5 fish species in lower trophic groups, opposite in direction to the top predator, including decreases in average size and longevity at Palmyra relative to Kiritimati. Overall, these results suggest that fishing may alter community structure in complex and non-intuitive ways, and that indirect demographic effects should be considered more broadly in ecosystem-based management.

Figure 1. Map of the Line Islands, including Palmyra and Kiritimati.

Figure 2. Size, maximum length and longevity between islands.

A) Sizes of the five study species at each island from survey data (mean length ±1 SE). B) Proportion of maximum sizes for each trophic group and all fish combined from the full species assemblage for each island, based on maximum sizes for each species from survey data (mean proportion of maximum length ±1 SE). C) Longevity for the 5 study species (T_max, mean of top quartile, ±95% CI). Data for Palmyra are in black, data for Kiritimati are in gray.

Figure 3. Size-at-age relationships for the 5 study species by island, with lines of best fit for VBGF parameters.

Inset plots for each species show 95% confidence ellipses for the parameters k and L_inf. Shading for histogram bars as in Fig. 2.