Mangroves and seagrass beds do not enhance growth of early juveniles of a coral reef fish

Abstract

Mangroves and seagrass beds have long been considered important nursery grounds for various species of juvenile reef fishes due to their higher abundances in these habitats compared to coral reefs. It is assumed that these putative nurseries provide juveniles with more shelter, higher food availability and higher growth and survival rates than on the reef. However, this nursery function is still ambiguous, and few experimental field studies have tested this hypothesis in these tropical habitats. In the present study, the growth rate of juvenile Haemulon flavolineatum and the availability of its preferred food were determined in seagrass, mangrove and coral reef habitats. It was hypothesized that somatic growth rates as well as the amount of preferred food are higher within these putative nurseries than on the reef (according to the nursery hypothesis). The growth of small juveniles (3.5 to 4.2 cm total length, TL) was studied at 2 Caribbean islands using in situ experimental growth cages. Gut content analysis of the caged fishes showed that Copepoda were by far the most consumed food items by juveniles in all 3 habitats. Copepoda in the plankton samples were more abundant on the reef than in the mangrove/seagrass habitats. Growth rates of fishes showed the same pattern: higher growth rates in length and weight (significant for Aruba, a trend for Curaçao), and a higher weight–length ratio on the reef compared to the mangrove/seagrass habitats. Based on these observations it appears that the coral reef would be a more suitable habitat for small juveniles, when not taking other factors such as predation risk into account. Nevertheless, the highest juvenile fish abundances are found in mangrove/seagrass nurseries where predation risk, but also growth rate, is lower. Therefore, it appears that a tradeoff exists between food abundance/growth rate and predation pressure/mortality risk, where fish select habitats that minimise the ratio of mortality risk to growth rate.