Discover Life -- Shorefishes of the Tropical Eastern Pacific -- Ecology
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Environments and habitats used by shorefishes

The fishes included in this book are mainly inhabitants of inshore environments, including not only rocky and coral reefs and adjacent habitats but also soft (sand and mud) habitats in estuaries and beaches as well as in water extending down the continental shelf. In general, tropical shore fishes are finely synchronised with their environment. Species on both reefs and soft bottoms exhibit precise habitat preferences that are dictated by a combination of factors including the availability of food and shelter, and various physical parameters such as salinity, depth, water clarity, currents, and wave action, as well as the precise nature of the bottom.

Tropical shore fishes generally exhibit a high degree of habitat partitioning. The tube blennies (family Chaenopsidae) provide a good example of the fine scale on which this principle operates. Many of the species live in a narrow zone which is determined by the presence of particular types of barnacle shells or polychaete worms whose empty shells and tubes are used for shelter and the site of egg deposition. Water depth is also an important partitioning factor, and there are numerous examples of reef fishes that have well defined, relatively narrow depth ranges. In the very broadest sense there are three main depth categories: shallow (0-4 m), intermediate (5-20 m), and deep (20 m+). Another bottom living fauna begins at about 100m. The depth limits of these zones may locally vary depending largely on the degree of shelter from wave effects. The shallow environment is typified by high wave action. In highly protected areas such as coastal bays, waves may exert their effect down to only a few cm. In contrast the effects of surface waves may sometimes be felt below 10 m on reefs exposed to heavy ocean swells. The intermediate depth zone generally has the greatest abundance of fishes and live corals. Here wave action is minimal, although currents are often strong.

Reef-fishes

Reef environments in the eastern Pacific can be broadly classified into two major categories: coastal mainland reefs and offshore or island reefs. Inshore or coastal reefs may be strongly influenced by freshwater runoff and resultant siltation. Underwater visibility on these reefs is relatively poor, sometimes nonexistent, particularly during the wet season when rivers are flowing at their maximum. Offshore reefs are found in clearer water environments and are less likely to be affected by freshwater runoff and consequent siltation. Underwater visibility on these offshore reefs usually is on the order of 20-30 m. Conditions are more suitable for coral, and indeed the best growth is found well away from the mainland. Coral growth is most abundant between about 5 and 15 m depth. In shallower water corals are inhibited by the pounding surge, and in deeper water by the much reduced penetration of light.

Although coral reefs represent the main habitat for reef fishes in most other parts of the vast Indo-Pacific tropics, they are poorly represented in the eastern Pacific. Although corals may be found on hard bottoms throughout most of the region reefs constructed by corals are relatively uncommon. These are concentrated in the equatorial part of the region, particularly around Panama, in the Gulf of Chiriqui, in western Panama. This area is not subjected to cold seasonal upwellings, unlike the nearby Gulf of Panama where coral reefs are more poorly developed. Because of the more stable thermal regime there is extensive development of certain hermatypic corals to depths of about 10 m. Coral diversity is relatively low, with only 41 species of reef building corals known from our region and coral reefs are constructed by a few species of Pocillopora, Porites and Pavona.

The majority of inshore fishes in the region likely to be encountered by divers and anglers, are associated with what can be broadly defined reef habitats. These include rocky shores and submerged rock reefs as well as coral reefs. About 40 % of the regional fauna are bottom living fishes found in association with reefs. Rock outcrops and ledges provide necessary shelter and a source of benthic invertebrates and algal growth, important dietary components for many reef fishes. Rocky reefs often have a dense covering of various types of seaweed, which provides additional shelter for a variety of small fishes. Some of the more prominent families found on rocky reefs include moray eels, snappers, grunts, damselfishes, wrasses, parrotfishes, blennies (three families), gobies, and surgeonfishes. Coral reefs cover only about 25km2 of our region, which represents only about 0.1% of the area of continental shelf that potentially could support coral growth.

Soft-bottom fishes

Another large and important segment of the eastern Pacific fauna is associted with soft bottom habitats which range from clean white sand in clear waters to soft thick mud, characteristic of turbid bays and estuaries. Although this habitat is often considered to be low in fish diversity, the total number of species utilizing this environment is surprising: about 40% of the species treated here live in such habitats. The soft bottom community includes such fishes as rays, snake eels, catfishes, lizardfishes, croakers, sand stargazers, and flatfishes.

Water-column fishes

The third most important group of fishes live in the water column above the substratum, over shallow reefs and soft-bottoms as well as the open ocean. These species constitute about 20% of the species included here. These include jacks, anchovies, herrings, and silversides, flyingfishes, tunas and billfishes, as well as a smattering of members of many other families.

Use of environments of differing salinities

Almost 70% of members of the shorefish fauna are found only in marine (i.e. full salinity) environments. Another 23.1% occur in estuarine (reduced salinity) as well as marine situations. Only 6.7% are non-marine species (which do not enter full salinity environments), and only 5.8% enter freshwater. A very small number (1.2%) are basically freshwater fishes that occasionally enter brackish water, or whose juveniles are encountered in brackish water.

Depth distribution pattern

Most of the regional fauna lives in fairly shallow water - 41% of species reach the surface, the depth ranges of 73% extend to within 5m of the surface, and only 3.5% do not come above about 50m. The lower limit of the depth ranges of 50% of the species terminate at about 30m, with only 32% occurring down to 50m and 19% below 100m.

Fishes diets

The great majority of our shorefishes are carnivorous to one extent or another: 80% of the species have such a diet. The next most abundant group of species consume plankton: 25% of the fauna include plankton in their diets, either exclusively or with a mix of non-planktonic animals. Bottom feeding omnivores and herbivore/detritivores are relatively uncommon, constituting only about 6% and 4% of the fauna, respectively. Items included in diets of carnivores range from tiny bottom living invertebrates such as molluscs and worms and crustaceans to sharks, whales and seabirds.

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