Body size and mortality rates in coral reef fishes: a three-phase relationship

Abstract

Body size is closely linked to mortality rates in many animals, although the overarching patterns in this relationship have rarely been considered for multiple species. A meta-analysis of published size-specific mortality rates for coral reef fishes revealed an exponential decline in mortality rate with increasing body size, however, within this broad relationship there are three distinct phases. Phase one is characterized by naive fishes recruiting to reefs, which suffer extremely high mortality rates. In this well-studied phase, fishes must learn quickly to survive the many predation risks. After just a few days, the surviving fishes enter phase two, in which small increases in body size result in pronounced increases in lifespan (estimated 11 d mm^-1). Remarkably, approximately 50% of reef fish individuals remain in phase two throughout their lives. Once fishes reach a size threshold of about 43 mm total length (TL) they enter phase three, where mortality rates are relatively low and the pressure to grow is presumably, significantly reduced. These phases provide a clearer understanding of the impact of body size on mortality rates in coral reef fishes and begin to reveal critical insights into the energetic and trophic dynamics of coral reefs.

Keywords: body size; cryptobenthic reef fish; growth; mortality; predation; recruitment.

Figure 1.

The relationship between body size (TL) and mortality rates (% day⁻¹) for 322 individual estimates from 156 coral reef fish species. Red points represent naive coral reef fishes (less than 48 h post-settlement), while blue points represent experienced fishes (more than 48 h post-settlement). The overall relationship appears to follow an exponential decline (r² = 0.36). (Online version in colour.)

Figure 2.

(a) The results of a two-way beta regression model revealed differences in the relationship between body size (TL) and mortality rates in naive recruits (red points and upper trend line) in phase one, and experienced fishes of comparable size (less than or equal to 54 mm TL; blue points and lower trend line; model interaction term p < 0.0001; electronic supplementary material, table S2). (b) During phase two, experienced fishes display a rapid decline in mortality rates with increasing size (solid blue points; slope =−0.10). An unconstrained segmental linear regression (r² = 0.80) identified the transition to phase three (open blue points) at x₀ = 43.12 ± 0.66 mm. (Online version in colour.)