Their Role in Fisheries Conservation

The three categories of marine protection in the Cayman Islands, the Environmental Zone, Marine Parks and Replenishment Zones perform many important functions, but perhaps one of their most important and less obvious roles is the protection of our local fisheries. Marine parks ensure that our local fisheries and marine resources continue to thrive by protecting a portion of the breeding stock from exploitation and limiting direct human impacts. To most of us the surrounding ocean seems boundless, its resources inexhaustible, and its ability to tolerate our activities unlimited. We are now beginning to realize that these perceptions are false: the ocean's resources do have limits and our activities can be devastating. In response to this many countries have established protected areas - marine parks - where human activities are regulated or prohibited. The concept of marine parks is simple: If protected from human interference, nature will take care of itself. The theory behind marine parks as fisheries reserves is not really new. Until recently, most reef fisheries were probably partly maintained by naturally protected areas: areas either too deep, too remote, or too difficult to locate easily. With improved fishing methods and more people fishing, the effectiveness of these natural refuges diminishes.

The ecology and life history of our reef organisms make them vulnerable to increased fishing pressure and other environmental impacts. Most reef species, including the conch and lobster, have a two-stage life cycle that consists of a pelagic (open water) egg or larval stage, and a demersal (bottom dwelling) juvenile or adult stage. The eggs and larvae are passively transported and dispersed as plankton by the ocean's currents. Depending on the species and location, eggs and larvae can drift from between one week to several months before larvae settle to bottom habitats. Once settled, juveniles and adults tend to remain in the same area for the rest of their life.

Reef creatures typically must produce millions of eggs and larvae, as survival at sea is generally very poor. Numbers at settlement from the planktonic stages can often vary by orders of magnitude from year to year due to the uncertainties in currents, weather, food availability and predation. This annual variability means that some years may have good recruitment years whilst other years can be very poor. However once settled, the reef organisms have a greatly increased chance of survival, and can live for many years, often decades. Some corals live for centuries.

The newly settled juveniles must allocate most of their food energy to growth and consequently reproduction is often delayed for several years. Once they reach adulthood their energy is largely allocated to reproduction and their growth rate slows dramatically, but in most cases, unlike humans, it doesn't stop all together and older fish continue to grow larger. Total egg production usually increases exponentially with body size. For example one 61 cm red snapper can produce as many eggs as 212 smaller 42 cm females. This size relationship is not only restricted to fish, but also conch, lobster and many other marine creatures, consequently a few older individuals may be extremely important to total egg production and population replenishment.

In natural conditions adults of exploited reef species are typically characterized by slow growth, low adult natural mortality, long life and large body size. Large body size is often an advantage as it helps in capturing prey and escaping predation. Unfortunately fishermen tend to selectively target and remove larger individuals because they provide more excitement, food and revenue than the smaller fish. This can have severe consequences for the population structure and recruitment of fishery resources. Excessive fishing can deplete the populations of certain species to numbers where there are too few eggs produced to replace the adult population. For species such as the groupers, which change their sex by switching from female to male with age, size-selective fishing can create a shortage of males to fertilize eggs. Even if fishing levels are acceptable for average conditions, a population could collapse after several years of unusually poor recruitment due to natural environmental events. Through poorly managed fishing many fisheries around the world have been depleted or collapsed entirely, such as the reef fisheries in Bermuda and Puerto Rico. Initial symptoms of overexploitation usually include a decline in average fish size and disappearance of larger species, and ultimately disruption of the marine ecosystem, and damage to a coastal area's overall economy.

Reef fishes are vulnerable to overfishing because they can be predictably located in time and space. Some species form large spawning aggregations at specific places and times, making them easily exploited. Aggressive behaviour, curiosity and inexperience with humans also make many species vulnerable to fishing.

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