Role of Mangrove Environment in the Life History of Marine Fishes

H. Role of Mangrove Environment in the Life History of Marine Fishes

By Will Heyman

Major Points of Chapter:

1. The life history strategy of many marine organisms includes an obligate period of dispersal in the plankton – ending in a period within mangroves.

2. Mangroves provide two distinct services for fishes: a) the prop roots constitute a protected habitat for larvae and early juveniles, and b) mangrove leaves form the basis of the detrital food web on which many fish depend.

3. There is a seasonal utilization of mangrove habitat divided into spatial and temporal niche spaces. Breeding events are followed by pelagic periods, recruitment events, and then blooms in predators, both zooplankton and nekton.

4. Ocean currents can control delivery of planktonic organisms into mangroves.

5. Mangroves form a crucial link in the life history of most of the commercially important reef organisms, and are thus economically linked to fishing and tourism industries.

Introduction: The life cycle of Marine Organisms

The life cycle of most marine fish includes an obligate period within mangrove ecosystems. Marine fish generally reproduce together, releasing large numbers of offspring, with little parental investment. These “spawning aggregations” result in mass spawning events, with fertilized eggs being released into the water. Within 24 hours, these floating eggs divide from a single cell and differentiate into tiny swimming larvae, which inhabit marine plankton. They follow ocean currents and eventually, if they encounter suitable habitat, will metamorphose into their juvenile forms within mangrove and seagrass habitats in a process called recruitment. The mangrove and seagrass habitats provide two important things for the juveniles: food and shelter. As juveniles grow into young adults they often continue to forage in mangrove and seagrass communities, until final migration back to open reef environments, where they will spawn and renew the cycle. The maintenance of marine biodiversity and the organisms of reef environments depend on the health of all the intact communities required in the life cycle of each organism and the connections between them.

Tropical Oceans are Deserts

Why are temperate oceans, and those oceans on western continental slopes green and dark while tropical areas, like the waters of the Caribbean Sea seaward of Belize, are turquoise blue and transparent? The darker waters are rich with phytoplankton, which blooms in response to the presence of sunlight and

The Importance of Mangrove Communities in Food Webs

Mangrove prop root communities are complex ecosystems surrounding the roots of red mangroves. Mangrove leaves, wood, propagules, flowers, bracts, and other organic materials fall continuously to the intertidal forest floor. These leaves and other litter cannot be digested by herbivores and are thus unavailable nutrient sources for higher trophic levels. When bacteria and fungi metabolize the leaf litter, however, it releases nutrients via a pathway that has been called the detrital food loop. The detritus is eaten by shrimp, mullet, and myriad smaller organisms within the mangrove prop root community and thus passed into the food web.

The Gray Snapper: An Example of the Connections Between Mangrove. Seagrass. and Coral-Reef Communities

The gray snapper, Lutjanus griseus, is one example of a species that depends on the intact links between these three communities to complete its life cycle. It spawns on the ocean side of reefs. The postlarvae then move out into the seagrass beds to feed and seek protection. Once the juveniles are at least 70 mm in length, they inhabit the mangrove prop-root community and migrate out into the seagrass beds every night to feed. When they become adults, they return to the coral reefs to live and reproduce. Shrimp, blue crabs, lobsters, and mullet are other examples of organisms that move to different habitats during their life cycle. The life histories of such species help illustrate the importance of maintaining the complex interactions between mangrove, seagrass, and coral reef communities.

Reef Fish Spawning Aggregations

Most commercially important reef fish species migrate great distances, and aggregate for spawning at specific times and locations. In Belize, 14 such aggregations have been recently located and documented by a group of national and international scientists and fishermen in Belize (Heyman et al. 2001). Recent findings suggest that these aggregation sites are all located at reef promontories, protruding edges of reefs that jut into deep water on the windward side of the barrier reef and atolls. It has been further documented that as many as 26 species (including most of the snappers and groupers) spawn at such sites, at various times of year, and at various times of the month. Thus, these sites are predictable in time and space. Using a combination of tools, including the knowledge of patriarch fishermen, aerial photographs, and satellite imagery, the location of potentially important spawning sites can be predicted. Careful underwater observation can help to document and monitor these sites, and determine their importance as spawning sites.

Since most of the reproductive output from any given species occurs at the brief time of the spawning aggregations, these times and places are crucially important in the life cycle of these

Transport of Gametes

Where do eggs and larvae go after they are released at spawning aggregations? Though the timing and location of spawning aggregations are beginning to be more accurately documented, the transport fate of released gametes is far less obvious. Larvae of snappers and groupers generally stay in the plankton for about 3 weeks, before they metamorphose into juveniles. It is generally believed that surface ocean currents transport the eggs, which float for 24 hours, until the larvae are able to swim. In their first few days, larvae are motile, but cannot swim far or fast, and do not begin to eat until about 4 or 5 days after hatching, when their mouth parts are developed. As the larvae mature, their swimming ability increases dramatically such that Pacific fish larvae are believed to be able to swim as far as 5 km during a single day. So, where do these young fish go? It is generally believed, as stated above, the young juvenile fishes wind up in shallow mangrove prop root communities, but this is poorly documented and not well understood.

Ocean and Nearshore Currents

Though the oceans appear on maps as large areas of blue, there are complex movements and dynamics within these systems. Ocean currents can transport the larvae and eggs of many species, from the site of their spawning, towards the eventual site of their recruitment. Currents can be studied using

a variety of modern tools including some instruments that are used in the water, and others that are categorized as remote sensing. Current meters can be either Lagrangian (moving with the current) or Eulerian (stationary meters that can measure currents moving by). These instruments can be complex, computer assisted and expensive electronic tools. Alternately, very simple tools, such as current drogues, can provide meaningful data at low cost. Remotely sensed data that can be useful in tracking ocean currents include measures of sea surface temperature, sea surface height, and chlorophyll content.

The currents of the Caribbean Sea are dominated by the existence of circulating current gyres that spin in circles as they move westward across the Caribbean Sea. Most gyres in the northern part of the basin are anticyclonic eddies, spinning clockwise, while those in the southern Caribbean are cyclonic eddies, spinning counterclockwise. These gyres move slowly across the ocean basin until they collide with the Belize Barrier Reef and Atolls. Currents around the reefs are driven by the presence and absence of these gyres. Though the gyres are stochastic in space and time, there is some repeatability to their movements and models of these currents are helping scientists and managers to understand the dynamics of circulation around the reefs. New multidisciplinary studies of larval release, transport, and recruitment require collaboration between biologists and physical oceanographers.

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