Marginal sinks or potential refuges? Costs and benefits for coral-obligate reef fishes at deep range margins

Abstract

Escalating climate-related disturbances and asymmetric habitat losses will increasingly result in species living in more marginal habitats. Marginal habitats may represent important refuges if individuals can acquire adequate resources to survive and reproduce. However, resources at range margins are often distributed more sparsely; therefore, increased effort to acquire resources can result in suboptimal performance and lead to marginal populations becoming non-self-sustaining sink-populations. Shifting resource availability is likely to be particularly problematic for dietary specialists. Here, we use extensive in situ behavioural observations and physiological condition measurements to examine the costs and benefits of resource-acquisition along a depth gradient in two obligate corallivore reef fishes with contrasting levels of dietary specialization. As expected, the space used to secure coral resources increased towards the lower depth margin. However, increased territory sizes resulted in equal or greater availability of resources within deeper territories. In addition, we observed decreased competition and no differences in foraging distance, pairing behaviour, body condition or fecundity at greater depths. Contrary to expectation, our results demonstrate that coral-obligate fishes can select high-quality coral patches on the deeper-reef to access equal or greater resources than their shallow-water counterparts, with no extra costs. This suggests depth offers a viable potential refuge for some at-risk coral-specialist fishes.

Keywords: body condition; coral reef fish; depth gradient; ecological costs; marginal habitats; refuge.

Figure 1.

(a) Interspecific similarities in territory size between a shallow-specialist (Chaetodon baronessa—red) and deep-generalist (C. octofasciatus—blue) obligate coral-feeding butterflyfish species, and intraspecific variation in territory size along a depth gradient for (b) the shallow-specialist species and (c) the deep-generalist. (d–f) Within-territory resource densities, showing (d) the interspecific similarities, (e) intraspecific variation along the depth gradient for the shallow-specialist and (f) depth variation for the deep-generalist. (g–i) Total secured resources within territories, showing (g) interspecific similarities, (h) intraspecific variation along the depth gradient for the shallow-specialist and (i) depth variation for the deep-generalist. Lines above bars in (a,d,g), represent statistically similar means. In regression plots, each variable is modelled against the median depth of territories; solid lines and straight dotted lines represent best fits, bands represent 95% confidence intervals and each data point represents a territory.

Figure 2.

Depth-related variation in competitor density and territorial maintenance effort. (a) The density of directly neighbouring conspecifics around territory perimeters. (b) The number of territorial interactions (insert shows shallow-specialist only), (c) the distance moved and (d) mean paring ratios. Solid lines represent the best fit for generalized linear models and bands are 95% confidence intervals.

Figure 3.

Relationships between body condition and depth. Metrics are: (a,f) total length; (b,g) relative body mass; (c,h) hepatosomatic index; (d,i) reproductive potential; (e,j) energy storage. Data points = individual fish. Closed circles = females. Open circles = males.