PRELIMINARY RESULTS ON THE TROPHIC DYNAMICS OF THE GULF OF NICOYA, COSTA RICA SCIENTIFTC NOTE

Trabajo recibido el 3 de mayo de 1984 y aceptado para su publicación el 20 de septiembre de 1984.

RESUMEN

INTRODUCCIÓN

The Costa Rican fishing industry harvests a moderate resource which yields no more than 25,000 annual metric tons per year. The Gulf of Nicoya an estuarine system bordered by mangrove forests - some altered and some threatened- is the country's most important fishing center. Shrimp trawlers sweep its floor searching for white shrimp and cause great mortality to other invertebrates and fishes. This anthropormorphic impact over the mangroves, the ichtiofauna and meiofauna threatens the stabílity of a system which is consídered a nursery area for some of the commercial ichthyofauna of the Gulf and some migratory species such as sharks (Madrigal, E., personal communication, 1983).

In tropical estuarine system with strong fishing pressure, environmental alterations such as deforestation of mangroves and high species diversity, a comprehensive view of the existing trophic dynamies is basic to understand the energy flow in the system and to achieve an adequate management of the commercially exploited species. In such a multispecies system, commercially important species may coexist with species of neglectable commercial value, but which nonetheless serve as potential prey for those harvested commercially. (Austin and Austin, 1971; Yáñez-Arancibia, 1978).

Stomach contents analysis in fishes is used to enlighten the prevailing dinamics –predator - prey, competition– between the members of a community.

This work representes a first attempt to assess, based on our data and information for other tropical estuarine systems, the trophic dynamies of the Gulf Nicoya, the main paths of energy flow through the system, and to identify research needs.

Data were collected monthly from March to November 1982, at 8 stations of the Gulf of Nicoya (Fig. 1). A bottom trawl was towed for 10 minutes at each station with a 10 meters headrope length, 4.5 cm streched mesh. The stations selected for this 10 month study were basically the same monitored during a four year cooperative research program of the Gulf of Nicoya between the University of Delaware and the University of Costa Rica.

The samples were transported to the laboratory in lee for analysis. Two hundred and ninety three fish were caught, identified, measured for total length and disected. Percentage composition of the contents by major taxonomic groups was estimated visually (Hyslop, 1980) with a B & L Stereo Zoom. Contents were identified to species level whenever possible. When material was partially digested the parts of each organism were utilized to indicate its presence i.e, fish spinal columns, pairs of shrimp chelipeds.

Fig. 1. Map of the Gulf of Nicoya showing the stations sampled during this study (1-8). Notation: N = Nicaragua; CR = Costa Rica; P = Panamá.

Table 1, gives the compositon by the taxonomic groups found in the stomach of the 21 species studied. Shrimp is a significatn component of contents of most of the species. Other species feed mainly on ophiurids and polychaetes. Isopisthus remifer, Cynoscion phoxocephalus and C. squamipinis showed a greater occurrence of fish in their diets. Three species, Anisotremus dovii, Stellifer chrysoleuca and Diapterus peruvianus had a relatively higher percentage of mollusks in their stomachs. It is difficult to assure if there is a preference for them or if they are accidentally taken while feeding.

Figure 2, shows the percentage contribution of each of the prey groups to the total found. Crustaceans are the dominant group and among them, shrimpare the most abundant component. Table 2 lists the shrimp species found in the stomachs. Trachipeneaus and Protrachypene are the commonest but many shrimp were not identified because their rostrums were missing. Although the results showed in figure 2 summarize the contributions of prey of all the disected specimens, other studies (Rutherford et al., 1982; Clairo and Lapin, 1971; Silva, 1974) show that is the case of Cynoscion nebolusus, Lutianus synagris and Epinephelus striatus crustaceans represented respectively, 43.5 %, 52.0% and 55.7% of the total stomach contents.

TABLE 1 PERCENTAGE COMPOSITION OF STOMACH (BY MAJOR TAXONOMIC GROUPS) OF FISHES FROM THE GULF OF NICOYA, COSTA RICA.

The faunal composition indicated here probably underestimates the real diet of the studied ichthyofauna, because our sampling schedule varied between 7:00 to 16:00 hours and was not based on peak feeding times. Other factors such as: a) fast digestion rates in tropical fishes (Reshetnikov et, al., 1974); b) the use of a trawl net to sample which provokes regurgitation in some fish; and e) not having considered changes in diet with size (Rodríguez, 1962; Austin and Austin, 1971; Rutherford et al., 1982) made it difficult to find the full array of prey organisms consumed. However, results found similar areas by other authors cited elsewhere in the text indicate that we have a good view of the prevailing feeding patterns.

Fig. 2. Percentage contribution of each major taxonomic group to the total number of organisms found in the stomachs analyzed. Data from the Gulf of Nicoya, Costa Rica. Notation: C=crustaceans (Squila=6.1 crabs=6.5%; shrimp=45.7%); F=fish; 0=Ophiurids, Po=Polichaetes; M=Mollusks; Pa=Parasites; NIM=Non identified matter.

Two aspects derived of the previous discussion deserve examination, 1) why crustaceans are the dominant prey item and 2) probable implications of their abundance as a prey item:

High abundance of crustaceans in the Gulf could lead to dominance in the stomachs of the fish species studied. At the eastern margin of the Gulf of Nicoya - which coincides with the sampled area in this study- shrimp represented annually, on the average, 80 % of the total number of invertebrates and 50 % of the total biomass (Maurer et. al., 1980). This argument may sound too simple to reflect the feeding dynamies of the studied species. However, excluding the energetie value of the individual prey consumed, crustacean abudance could considerably reduce the searching time of the predator and maximize energetie consumption per unit time.

TABLE 2 SHRIMP SPECIES FOUND IN STOMACHS OF THE STUDIED ICHTHYOFAUNA OF THE GULF OF NICOYA, COSTA RICA

TABLE 3 ORGANISMS FOUND IN STOMACH CONTENTS OF THE ICHTHYOFAUNA FROM THE GULF OF NICOYA, COSTA RICA. CONTENTS WERE IDENTIFIED TO ESPECIES LEVEL WHENEVER POSSIBLE

Of the identified prey listed in Table 3 mollusks are the more diverse. However, their abundance in stomachs of all species combined is less than 4% of the total (this could be explained by using the reverse argument of the one for shrimp). Maurer et al. (1980) states that gastropods represent, on the average, 0.35 % of the total number of invertebrates foud and 6% of the total biomass.

Protection and food availability determine a substrate as a suitable habitat for shrimp (Williams, 1958; Turner, 1977; Bradford, 1981). The properties of the substrate depend substancially on river runoff. Mangrove leaves are one of the main components of debris transported by rivers draining mangrove channels and estuaries; and transported whole, broken or as detritus, partially condition the quality of the substrate in terms of softness, organic content and partiele size. This organic detritus provides a substrate for microbes, bacteria, yeasts and fungi which are consumed by shrimp (Evink, 1975; Velez, 1978; Yáñez-Arancibia, 1978; Prahl, 1980; Odum, 1980) Other crustaceans such as crabs also depend directly on this substrate as part of their diet and indirectly since they prey upon crabs annelids, and mollusks associated with andior feeding on the detritus (Yáñez-Arancibia, 1978; Velez, 1978). Based on this review and our results, we could hypothesize that approximately 50 % of the prey to sustain the studied ichthyofauna depend on aloctonous material coming from mangroves swamps next to the fishing areas. Although this discussion emphasizes the importance of mangrove forests for the perpetuation of crustacean resources, we should not forget that the sum of several variables such as rainfall, recruitment, and environmental change at the nursery areas, condition the grater or lesser abundance of crustaceans and other invertebrates (Gunther and Edwards, 1967; Saldaña and López, 1969). Figure 3, shows a trophic pyramid using results of Table and following Yasuda's criteria (In: YáñezArancibia, 1978) for its construction. Eleven species (52.3%) are found to be omnivores and feed on three or more of the prey groups listed in Table 1. Seven species (33.4 %) were considered second order carnivores and feed mainly on shrimp and fish and ocasionally on squid (i.e. C. albus). The other three species (14.3 %) were considered first order carnivores and feed specially on fish and shrimp. C. phoxocephalus shows a 3.4 % of mollusks (squid) in the stomachs but because of the high percentage of fish consumption was considered a first order carnivore. We are aware that this figure represents a broad generalized picture of the trophic scheme of the Gulf of Nicoya. There are many piscivorous fishes such as native sharks from the Gulf of Nicoya, migratory hammerheads which use the Gulf as a nursery area, carangids, lutjanids, scombrids and larger specimens of some of the studied species (C. phoxocephalus. C. squamipinis, Centropomus unionensis among others) which were not captured by the shrimp trawling net probably because of their fast swiming.

Fig. 3. Trophic pyramid based on results stomach content analysis of the ichthyofauna from the Gulf of Nicoya, Costa Ríca. Notation: FO = first order carnivores; SO = second order carnivores; OMV = omnivores.

In a lagunar or estuarine system such the Gulf of Nicoya there are two main types of food chains, one based on grazing and the other on detritus feeding. First order consumers include most zooplankters, fish, shrimp and crabs. This diverse group comprises hervivores, detritivores and omnivores (Day and Yáñez-Arancibia, 1982). The crustaceans, annelids, mollusks and other detritivores and omnivores invertebrates found as prey in this study can be considerer as first order consumers of the Gulf of Nicoya. These invertebrates as well as omnivorous fishes depend on some energetic source other than the phytoplankton. Here, the detrital chain, mangrove and other trees and plants which supply organic detritus, detritus consumers, omnivores and carnivores could constitute such additional route. It should be remembered that even when the detritus chain is ecologically and economically very important for a system such a Gulf of Nicoya, the role of phytoplankton should not be overestimated, specially regarding ichthyoplankton feeding dynamies (Odum et al., 1982).

The Gulf of Nicoya is fished intensively by both the artisanal and the industrial commercial fishery and shrimp trawling is specially intense in the outer Gulf (Fig. 1). Additionally, mangrove cutting for land reclamation and charcoal have been slowly modifying these marginal ecosystem of the Gulf. If our results on prey consumption are taken for all species combined, we identify two inmediate research priorities.

a. To depen our understanding of the trophic dynamies of the Gulf where it appears that part of the commercial ichthyofauna rely over 50 % on crustaceans (specially shrimp) as a prey item, and.

b. To highlight the role of mangrove ecosystems in the eco logy of the Gulf of Nicoya.

We wish to express our gratitude to Eduardo Madrigal from their Fisheries Department for his help with fisheries data; to David Stevenson from the University of Maine and to Bradford Brown from the Northeast Fisheries Center, for reviewing a preliminary copy of this paper; to Carlos Villalobos, Ana Dittel, William Bussing and Myrna López who identified most invertebrates and fish and to Norma Bermúdez who typed it. This work was funded by grant No. 02071424 from the Vicerrectory for Research of the University of Costa Rica.

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