Simple ecological trade-offs give rise to emergent cross-ecosystem distributions of a coral reef fish

Abstract

Ecosystems are intricately linked by the flow of organisms across their boundaries, and such connectivity can be essential to the structure and function of the linked ecosystems. For example, many coral reef fish populations are maintained by the movement of individuals from spatially segregated juvenile habitats (i.e., nurseries, such as mangroves and seagrass beds) to areas preferred by adults. It is presumed that nursery habitats provide for faster growth (higher food availability) and/or low predation risk for juveniles, but empirical data supporting this hypothesis is surprisingly lacking for coral reef fishes. Here, we investigate potential mechanisms (growth, predation risk, and reproductive investment) that give rise to the distribution patterns of a common Caribbean reef fish species, Haemulon flavolineatum (French grunt). Adults were primarily found on coral reefs, whereas juvenile fish only occurred in non-reef habitats. Contrary to our initial expectations, analysis of length-at-age revealed that growth rates were highest on coral reefs and not within nursery habitats. Survival rates in tethering trials were 0% for small juvenile fish transplanted to coral reefs and 24-47% in the nurseries. As fish grew, survival rates on coral reefs approached those in non-reef habitats (56 vs. 77-100%, respectively). As such, predation seems to be the primary factor driving across-ecosystem distributions of this fish, and thus the primary reason why mangrove and seagrass habitats function as nursery habitat. Identifying the mechanisms that lead to such distributions is critical to develop appropriate conservation initiatives, identify essential fish habitat, and predict impacts associated with environmental change.

Fig. 1

Conceptual models of underlying mechanisms that explain the use of coastal nursery habitats by juvenile fish in terms of fish growth (dashed lines) and survival (solid lines). a Growth rate is higher in nursery habitats (i.e., the bay) while survival is equal among bay and reef, b survival is higher in the bay while growth is equal among bay and reef, or c both survival and growth are higher in the bay than on the reef (the most commonly assumed scenario for coral reef fishes in nursery habitats). d A potential trade-off between predation risk and growth rates in the two ecosystems

Fig. 2

Distribution of Haemulon flavolineatum in the bay and channel areas of Spanish Water Bay, and on the fringing coral reef on Curaçao based on a 5-month census period. Gray indicates land, white indicates sea, and the hatched area indicates coral reef. Relative fish densities are represented based on 4-cm size classes (FL) in pie charts

Fig. 3

Von Bertalanffy growth curves for Haemulon flavolineatum in the bay (black boxes) and channel (gray boxes) areas of Spanish Water Bay and on the reef (open boxes) on Curaçao. Box plots for each age class are jittered for visibility. Note that the black box for year 0 fish and the gray box for year 3 fish are absent as n = 1 in each case. Von Bertalanffy parameters for H. flavolineatum were L _∞ = 14.1, t ₀ = −0.53, and k = 0.69 for the bay; L _∞ = 14.3, t ₀ = −1.05, and k = 0.39 for the channel; and L _∞ = 18.7, t ₀ = −0.64, and k = 0.52 for the coral reef. The number of otoliths sampled per area per age class is provided above the graph

Fig. 4

Survival from predation after 90 min in tethering trials (mean + SE) of Haemulon flavolineatum for three different life stages in the bay (black bars) and channel (gray bars) areas of Spanish Water Bay and on the fringing coral reef (open bars) on Curaçao. Size ranges of tethered fish were chosen to represent the following life stages: recently settled fish (2.4–4.5 cm FL), approximate size at which fish start migrating to reefs (8.1–12.0 cm FL), and adult fish commonly found on reefs (13.8–16.9 cm FL). Within each size class, different letters indicate significant differences (P ≤ 0.05) among areas based on a logistic regression. No fish 2.4–4.5 cm FL survived the tethering trails after 90 min on the reef, while all fish 13.8–16.9 cm FL survived the 90-min tethering trials in the bay. The number of successful tethering trials per area per size class is provided above the graph

Fig. 5

Gonadosomatic index, GSI (mean + SE) of Haemulon flavolineatum per 4-cm size class FL in the bay (black bars) and channel (gray bars) areas of Spanish Water Bay and on the reef (open bars) on Curaçao. Within each size class, different letters indicate significant differences (P ≤ 0.05) among areas based on a one-way ANOVA followed by a Hochberg’s GT2 post hoc comparison or a non-parametric Kruskal–Wallis test followed by a Games Howell post hoc comparison. The number of fish sampled per area per size class is provided above the graph