Encounter with mesoscale eddies enhances survival to settlement in larval coral reef fishes

Abstract

Oceanographic features, such as eddies and fronts, enhance and concentrate productivity, generating high-quality patches that dispersive marine larvae may encounter in the plankton. Although broad-scale movement of larvae associated with these features can be captured in biophysical models, direct evidence of processes influencing survival within them, and subsequent effects on population replenishment, are unknown. We sequentially sampled cohorts of coral reef fishes in the plankton and nearshore juvenile habitats in the Straits of Florida and used otolith microstructure analysis to compare growth and size-at-age of larvae collected inside and outside of mesoscale eddies to those that survived to settlement. Larval habitat altered patterns of growth and selective mortality: Thalassoma bifasciatum and Cryptotomus roseus that encountered eddies in the plankton grew faster than larvae outside of eddies and likely experienced higher survival to settlement. During warm periods, T. bifasciatum residing outside of eddies in the oligotrophic Florida Current experienced high mortality and only the slowest growers survived early larval life. Such slow growth is advantageous in nutrient poor habitats when warm temperatures increase metabolic demands but is insufficient for survival beyond the larval stage because only fast-growing larvae successfully settled to reefs. Because larvae arriving to the Straits of Florida from distant sources must spend long periods of time outside of eddies, our results indicate that they have a survival disadvantage. High productivity features such as eddies not only enhance the survival of pelagic larvae, but also potentially increase the contribution of locally spawned larvae to reef populations.

Keywords: mesoscale eddies; otolith microstructure; population connectivity; reef fish settlement; selective mortality.

Fig. 1.

Map of study area depicting average location of mesoscale eddies in the Straits of Florida during sampling. Two eddies were sampled on each of three oceanographic cruises conducted in June 2007, August 2007, and June 2008. Nearshore sampling sites in the lower Florida Keys at AS (American Shoal) and LK (Looe Key) reefs where late-stage larvae and juveniles were collected are denoted by red triangles, and the approximate location of the Florida Current is shown in blue.

Fig. 2.

Daily growth (otolith increment width) at each day after hatch for ED, eddy; NE, non-eddy; and SUR, surviving late-stage larvae captured in light traps (C. roseus) or juveniles collected on the reef (T. bifasciatum) from cohorts sampled in June 2007 (A), August 2007 (B), and June 2008 (C). Error bars (± SE) are shown every five increments for reference. ŧ = significant result (P < 0.001–0.05) in one-way ANOVAs (Table 1). Sample sizes (n) are listed in Table 1.

Fig. 3.

Kernel density estimates of size-at-age (otolith radius) distributions at 15 dph in ED, eddy; NE, non-eddy; and SUR, surviving late-stage larvae captured in light traps (C. roseus) or juveniles collected on the reef (T. bifasciatum) from cohorts sampled in June 2007 (A), August 2007 (B), and June 2008 (C). Sample sizes (n) are listed in Table 1.

Fig. S1.

Mean temperature (± SE) at ED, eddy (green bars), and NE, non-eddy (blue bars) stations across depth bins sampled for ichthyoplankton in June 2007 (A), August 2007 (B), and June 2008 (C).

Fig. 4.

Mean growth (± SE) of young and old age groups for T. bifasciatum larvae collected inside (ED) and outside (NE) of eddies from cohorts sampled in June 2007 (A) and August 2007 (B). Comparisons among groups were made by using one-way ANOVAs for each of three periods of mean growth (6–10 dph, 11–15 dph, and 16–20 dph), and significant results (P < 0.001–0.05) are denoted with lowercase letters. Sample sizes (n) for each group are denoted in white on the corresponding bar.