Altered tropical seascapes influence patterns of fish assemblage and ecological functions in the Western Indian Ocean.

The arrangement and composition of habitats within landscapes and fine-scale habitat characteristics influence community structure and ecological processes. These aspects can be altered by anthropogenic activities, thus influencing associated assemblages. Farming of macroalgae is a common practice in tropical settings and alters the natural composition of seascapes by introducing monoculture patches. The farmed macroalgae may also differ in palatability compared to naturally-occurring macroalgae, influencing herbivory. This study assessed how these farms may differ from natural macroalgal beds in terms of habitat heterogeneity, fish assemblages, and herbivory. We surveyed fish assemblages and deployed macroalgal assays within macroalgal beds, farms and at varying distances from these habitats near Mafia Island, Tanzania. Fish composition and herbivory differed between the habitats likely due to different macrophyte species richness, underlying hard substrate in natural macroalgal beds, and high abundance of browsers nearby the farms. Additionally, fish assemblage patterns and herbivory were not consistent across the seascapes and varied with distance from the focal habitats possibly due to the presence of other habitats. The results suggest alterations of seascapes by farming practices may have consequences on fish assemblages and the ecological functions performed, thus positioning of farms should be carefully considered in management and conservation plans.

Life-history studies by non-lethal sampling: using microchemical constituents of fin rays as chronological recorders.

Chemical properties of fin rays were investigated in nine fish species to test whether life-history characteristics can be analysed using a non-lethal and minimally invasive methodology. Fish specimens from public aquariums were acquired after fishes died in captivity. Analyses concentrated on exploring the differences between the wild and captive life periods of each fish, which were known from aquarium records. Differences between the two life periods were observed in both the trace-element and stable-isotope compositions of the chemical matrix of the fin ray. Trace-element concentrations in fin rays were compared with those in otoliths using measures of resolved variance and cross-correlation to test the assumption of conserved matrices in the fin ray. Divalent ions and positively charged transition metals (i.e. Fe and Co) had strong associations between the two structures, suggesting conservation of material. Stable-isotope values of δ13 C and δ15 N differed between the wild and captive life periods in most of the fishes, also suggesting conserved matrices. δ13 C and δ15 N were derived from the organic matrix within the fin ray, which may present a stable-isotope chronology. Future studies can use these chronologies to study diet and movement trends on a temporal scale consistent with the entire lifetime of an individual.

Using post-settlement demography to estimate larval survivorship: a coral reef fish example.

Many species have multi-stage life cycles in which the youngest stages (e.g., larvae) are small, dispersive, and abundant, whereas later stages are sessile or sedentary. Quantifying survival throughout such early stages is critical for understanding dispersal, population dynamics, and life history evolution. However, dispersive stages can be very difficult to sample in situ, and estimates of survival through the entire duration of these stages are typically poor. Here we describe how demographic information from juveniles and adults can be used to estimate survival throughout a dispersive larval stage that was not sampled directly. Using field measurements of demography, we show that detailed information on post-settlement growth, survival, and reproduction can be used to estimate average larval survivorship under the assumption that a typical individual replaces itself over its lifetime. Applying this approach to a common coral reef fish (bicolor damselfish, Stegastes partitus), we estimated average larval survivorship to be 0.108% (95% CI 0.025-0.484). We next compared this demography-based estimate to an expected value derived from published estimates of larval mortality rates. Our estimate of larval survivorship for bicolor damselfish was approximately two orders of magnitude greater than what would be expected if larval mortality of this species followed the average, size-dependent pattern of mortality inferred from a published sample of marine fishes. Our results highlight the importance of understanding mortality during the earliest phases of larval life, which are typically not sampled, as well as the need to understand the details of how larval mortality scales with body size.