Why do juvenile fish utilize mangrove habitat?

Study Number:

36

Author:

P. Laegdsgaard & C. Johnson

Abstract:

Three hypotheses to discern the strong positive association between juvenile fish and mangrove habitat were tested with field and laboratory experiments. Artificial mangrove structure in the field attracted slightly more juvenile fish than areas without structure. Artificial structure left to accumulate fouling algae attracted four-times the total number of juvenile fish than areas without structure or areas with clean structure. Community composition of fish attracted to structure with fouling algae was different when compared with areas with no structure or clean structure; five species were attracted by structure with fouling algae whilst two species were associated with structure regardless of fouling algae. Algae were linked to increased food availability and it is suggested that this is an important selection criteria for some species. Other species were apparently attracted to structure for different reasons, and provision of shelter appears to be important. Predation pressure influenced habitat choice in small juvenile fish in laboratory experiments. In the absence of predators, small juveniles of four out of five species avoided shelter but when predators were introduced all species actively sought shelter. Large fish were apparently less vulnerable to predators and did not seek shelter when predators were added to their tank. Feeding rate was increased in the mangrove habitat for small and medium-sized fish compared with seagrass beds and mudflats indicating increased food availability or foraging efficiency within this habitat. Larger fish fed more effectively on the mudflats with an increased feeding rate in this habitat compared with adjacent habitats. The most important aspect of the mangrove habitat for small juvenile fish is the complex structure that provides maximum food availability and minimises the incidence of predation. As fish grow a shift in habitat from mangroves to mudflat is a response to changes in diet, foraging efficiency and vulnerability to predators.

Main Results and Conclusions:

• “Hypothesis 1: juvenile fish are attracted to habitats of high structural heterogeneity”(233):
  • In the absence of predators: “all size classes of Sillago avoid shelter and spent more time in unstaked areas of experimental tanks than expected from random movement (Table 1). Similar to the results for Sillago, Atherinomorus ogilbyi clearly avoided shelter whereas Liza argentea was highly associated with staked areas (Table 1)”(238).
  • “The results in this study indicate that increased structural heterogeneity alone is insufficient to account for the strong association of large numbers of juvenile fish with mangrove forests. Although adding artificial mangrove structure to areas in the field significantly increased the total number of fish caught compared to unstructured areas, the increase was relatively slight”(246).
• “Hypothesis 2: juvenile fish seek shelter to reduce the risk of predation”(235):
  • “In the laboratory, shelter was actively avoided by six species of juvenile fish (Gerres ovatus, Atherinomorus ogilbyi, Ambassis marianus, Acanthopagrus australis ,Sillago  spp. and Liza argentea) during both night and day in 24-h trials in the absence of predators. In shorter-term experiments, Atherinomorus ogilbyi and Sillago spp. also avoided structure. Liza argentea showed a higher affinity for structure although still ventured away from structure in the absence of predators. When predators were added to the tanks, shelter was sought actively by all three species and the incidence of sorties into the open became fewer…decreased risk of predation appears to be an important factor underpinning the preferred use of mangrove habitats by juvenile fish”(248).
• “Hypothesis 3: habitat selectivity by juvenile fish reflects differential food availability”(237):
  • “Feeding rates of small and medium-sized fish were significantly greater in the mangroves compared with mudflat and seagrass habitats… While these results support the third hypothesis that the effective availability of food for small fish is greater within the mangrove habitat than in adjacent areas, it must be recognised that foraging gains are often balanced against the risk of predation… Results from this study show clearly that small juvenile fish in mangrove habitats have the benefit of both increased acquisition rates of food and increased protection. The large amounts of algae covering the pneumatophores” –respiratory root structure- “provide shelter and food for many invertebrates that are food for juvenile fish”(249).
• In conclusion: “Mangrove forests provide structure at an intermediate scale in which capture of invertebrate food prey by juvenile fish species appears optimal and risk from piscivorous predators is reduced. In other habitats within the estuary, such as seagrass beds, there is equal protection from predators but foraging success is reduced and therefore seagrass beds are less suitable for post-larval fish. With increased size, juvenile fish switch to mudflat habitats as their foraging success in mangroves is reduced (presumably because the complex structure of the mangrove forests becomes restrictive of foraging) and the fish become less vulnerable to predators and are able to forage in relative safety on the more open mudflats”(250).