8 SURVEYS OFF THE PACIFIC COAST OF CENTRAL AMERICA

8.1 INTRODUCTION

Survey objectives and effort

A programme of surveys with the DR. FRIDTJOF NANSEN of the Pacific shelf from the southern border of Colombia to the Gulf of Tehuantepec in Mexico was organized in 1987 within the framework of the UNDP/FAO Project GLO/82/001. The work was planned through the “Working Group on Fisheries Research in the Region of Central America and Panama” organized jointly by FAO and the Latin American Organization for Fisheries Development, OLDEPESCA. During 1987 four complete surveys of the study area, each of some six weeks duration, were made to cover (eventual) seasonal changes.

Cruise Reports, to outline the work done and present the main findings were issued after each survey (IMR, 1987c to 1987n). A final report was submitted in draft version to a Technical Consultation held in Costa Rica in May 1988 and subsequently issued in English and Spanish (Strømme and Sætersdal, 1988a and b). This chapter gives a brief review of the surveys and the findings.

The agreed general objectives of the programme were set out as follows:

• describe the composition, distribution and abundance of small pelagic- and demersal fish, squid and crustaceans on the shelf and upper slope to evaluate the potentials of these stocks;

• conduct taxonomic studies for the purpose of preparing a regional guide to the marine resources;

• conduct hydrographic investigations and prepare charts of bottom type based on echosounder observations;

• provide data on catch rates for specific selected gears and areas including deep-water shrimp and langostino on the slope and jigging for giant squid off the shelf.

Surveys took place in February-March, May-June, August-September and November-December 1987. The total survey effort comprised about 170 days of research work at sea with about 27,000 nmi survey distance and some 1,100 fishing stations. The mean degree of coverage with reference to the acoustic investigations was 11, which is not very high. Figure 8.1 shows as an example the course tracks and stations worked in one of the surveys off Panama.

Small pelagic schooling fish were investigated using acoustic echo integration combined with fishing with bottom and mid-water trawls for identification and sampling.

The demersal resources were investigated with a bottom trawl survey programme where most fishing trials were made in pre-located positions so that the results formed the basis for a swept-area analysis of the composition and abundance of the species.

Figure 8.1 Example of course tracks and stations in survey off Panama, February 1987

The various types of shrimp resources could not be covered in an appropriate way within the trawl survey programme. This would have necessitated a concentration of effort both seasonally and in the various special shrimping grounds which would have precluded most of the other survey objectives. For the inshore shallow-water penaeid shrimps the survey data only provide records of incidental catches. For shrimp at intermediate depth, such as the crystal shrimp (Penaeus brevirostris), the survey results may for some areas give indications of catch rates and seasons. A similar consideration applies to the shrimps in deep-water beyond 150 m, mainly kolibri shrimp (fidel) (Solenocera agassizii) and nylon shrimp (cabezon) (Heterocarpus vicarius).

Among the other crustaceans, a special effort was made to cover the resources of langostino (Pleuroncodes planiceps) by area, season and depth.

Ecological and faunistic studies

An analysis of the hydrographical data is available in Strømme and Sætersdal (1988a). The main finding regarding fishery oceanography was the confirmation of the disruption of the stable tropical conditions of the surface layers by the well-known seasonal upwellings in the Gulfs of Panama, Papagayo and Tehuantepec caused by strong winds blowing through passages in the mountain ranges between the Atlantic and the Pacific. Conditions of oxygen deficient bottom water (< 1 ml/l) were generally found on the outer shelf from about 100 m depth.

The type of bottom on the shelf and in the slope was observed along the survey tracks based on examination of the echograms and recorded by four categories: smooth even bottom, relatively smooth but uneven bottom, rough bottom and very steep bottom. Most parts of the shelf proper were smooth, but the slope was often rough and steep (Strømme and Sætersdal, 1988a).

Participation in the surveys of taxonomists appointed by FAO Department of Fisheries represented the start of the preparation a species identification guide for fishery purposes (Fischer et al., 1995).

Based on the survey data the demersal fish assemblages in the region were analysed and described (Strømme and Sætersdal, 1988a and Bianchi, 1992a). These studies showed that depth and the associated physical oceanographic conditions, were the main factors in determining the composition of the different species groups.

1. The catches from the deeper continental shelf and upper slope, where oxygen levels were very low (< 0.5 ml/l), were characterized by a low faunal diversity and species adapted to live in almost anoxic conditions.

2. Another group could be identified as intermediate shelf-dwellers, below the thermocline but in waters with higher oxygen content than that found in the deeper shelf and upper slope areas. Many of the species dominant in this part of the shelf were also found, although in smaller quantities, in shallower and/or deeper waters.

3. Finally, another major group could be defined as shallow water or littoral, with species usually encountered above the thermocline and in oxygen rich waters.

Within each of the three main groups, changes in species composition occurred with changes in type of substrate. In shallow coastal waters, the presence of estuaries and brackish waters strongly influenced species composition.

The combined results of the four surveys will be presented below on a country-by-country basis.

8.2 COLOMBIA, 1987

The Pacific shelf of Colombia is narrow, on average about 20 nmi. There is little variation through the year in the vertical structure of the water masses with a well defined thermocline between about 50 and 100 m of depth. There is some variation in surface salinity resulting from river runoffs in the rainy season. The northernmost part of the Colombian shelf and slope may be influenced by the seasonal upwelling system of the Gulf of Panama.

Pelagic fish

Small pelagic fish was found well inshore above the thermocline and distributed in patches along the coast and with the densest aggregations in the south. The main components were thread herring, anchovy, Carangidae (mostly bumper) with some lookdown and big-eye scad, and smaller amounts of sierra and barracuda.

The distribution of the pelagic fish was similar in all surveys and Figure 8.2 shows as an example that of the October-November survey. Highest densities were found in the south and the inshore surface assemblage here was dominated by the Pacific anchoveta (Cetengraulis mysticetus) and thread herring (Opisthonema libertate) found in school areas and with Pacific bumper (Chloroscombrus orqueta) closest inshore. Other accompanying Carangidae were lookdowns (mostly Selene peruvianus) and bigeye scad (Selar crumenophthalmus) with some green jack (Caranx caballus) and bright leatherjack (Oligoplites refulgens). Among the larger predators in this system were the sierra (Scomberomorus sierra), barracuda (Sphyraena ensis) and sharks, mostly hammerheads (Sphyrna spp.).

Figure 8.2 Colombia: Distribution of pelagic fish as recorded by the acoustic system in the October-November survey

A different fauna of pelagic fish was found below the thermocline from about the Gulf of Cupica northwards with hairtail (Trichiurus nitens), scad (Decapterus macrosoma) and argentine (Argentina aliceae). These species may also be present in deeper waters further south, but the very steep slope there prevented observations and sampling. The most interesting of these species is the scad which was found in some abundance in the deeper offshore parts of the shelf off Panama.

Demersal resources

Demersal fish was found mostly in the 20–100 m depth range along the coast and with highest catch rates in the south. About half of the catches consisted of small-sized species of no commercial interest. Among the potentially commercial fish, butterfish was most common, followed by mostly small-sized seabasses, sharks, snappers and grunts. The bottom trawl hauls in deeper waters in the north gave some high catch rates of hairtails and argentine.

The main demersal fish species found were butterfishes (Peprilus medius and P. snyderi), rose threadfin bass (Hemanthias signifer), catfish (Bagre panamensis), lizardfish (Synodus evermanni), spotted rose snapper (Lutjanus guttatus), Panama grunt (Pomadasys panamensis), argentine (Argentina aliceae) and widespur seabass (Diplectrum euryplectrum).

The catch rates in the bottom trawl were low with an overall mean of about 130 kg/h for all commercial families.

The nylon shrimp (Heterocarpus vicarius), colibri shrimp (Solenocera agassizii) and crystal shrimp (Penaeus brevirostris) were the most abundant shrimp species, all at depths beyond 50 m. The mean catch rates were low, 2–12 kg/h, but reached 150 kg/h for nylon shrimp in the north. Various penaeid shrimps inhabit the 0–50 m bottom depth range, but only incidental catches were obtained of these.

Dart squid (Loliolopsis diomedeae) appeared in the catches in somewhat deeper hauls, 50–100 m and beyond in various locations, but especially on the southern shelf where catches up to 190 kg/h were obtained in April. The catch rates reached a mean of about 50 kg/h with a maximum of nearly 200 kg/h. The data from Panama indicated a clear annual cycle in the availability of this squid with a maximum in the first part of the year and near-absence from August to November. A seasonal change is likely also for Colombia. The dart squid could only be fished in bottom trawl during daytime as it lifts off the bottom at night.

Biomass estimates

Table 8.1 shows the estimated standing biomass for the groups of resources and with a rough allocation on the most important species or sub-groups.

Table 8.1 Colombia: Estimated standing biomass by resource groups

The total biomass was thus estimated at about 100,000 tonnes and this gives a mean density for the shelf (0–200 m) of 18 t/nmi². If the estimated amount of non-commercial demersal fish of about 24,000 t is included, the density is 22 t/nmi². This is a level of density found in many tropical countries with similar ecological conditions (see Chapter 10).

8.3 PANAMA, 1987

The shelf of Panama can conveniently be separated into two main parts: the wide and extensive Gulf of Panama in the east and the western coast, with the Gulfs of Coiba and Chiriqui (Fig. 8.1). The Gulf of Panama is known as an important fishing area where seasonal upwelling and perhaps also river runoffs create conditions for higher productivity. Seasonality is evident in the hydrographic environment especially in the deeper parts of the Gulf, with important fluctuations in temperature and oxygen content near the bottom. In addition, there are also seasonal changes in the surface layers with a lifting and weakening thermocline in the upwelling season.

8.3.1 The Gulf of Panama

Pelagic fish

As an example, Figure 8.3 shows the distribution from the August survey. A general feature for this area is that aggregations of high densities were found throughout the year around the shores of the Gulf, while in the central and deeper parts, fish was only recorded in May and August, with a nearly complete absence in February and November, probably caused by the low oxygen levels near the bottom in this season.

Figure 8.3 Panama: Distribution of pelagic fish in the August 1987 survey

The assemblage of fish found above the thermocline, which seems to prefer depths of less than 50 m was dominated by thread herring (Opisthonema libertate) and anchovies (Anchoa sp. and Cetengraulis sp.) found in school areas in which schools of bumper (Chloroscombrus orqueta) were usually also frequent. Frequent predators mixed with these schools were sierra (Scomberomorus sierra), barracuda (Sphyraena ensis), sharks and various demersal fish.

The fauna below the thermocline in the middle offshore part of the Gulf, which is only present in significant quantities in May and August, was dominated by scads (Decapterus macrosoma) mixed with some round herring (Etrumeus teres) and various demersal fish.

Demersal resources

The demersal fish fauna of the Gulf of Panama can be grouped by three main habitats:

• nearshore to about 50 m bottom depth;
• offshore from about 50 m to the shelf edge;
• the slope, from the shelf edge and into the deep-sea.

The seasonally fluctuating hydrographic regime gives varying environmental conditions in the offshore shelf region, mainly through changes in oxygen content.

The main demersal fish species found in the Gulf of Panama were butterfishes (Peprilus medius and P. snyderi), rose threadfin bass (Hemanthias signifer), Pacific red snapper (Lutjanus peru), lizardfish (Synodus evermanni) and widespur seabass (Diplectrum euryplectrum). Frequent in the catches, but with less abundance, were Panama grunt (Pomadasys panamensis), yellow bobo (Polydactylus opercularis) and bigscale goatfish (Pseudupeneus grandisquamis).

The 0–50 m depth zone was relatively poor in demersal fish, compared to the deeper strata. The main species in the shallow waters were butterfishes, spotted rose snapper (Lutjanus guttatus) and the Panama grunt.

The main species in the 50–100 m bottom depth stratum were butterfish, lizardfish, searobin (Prionotus quiescens) and the widespur seabass.

In the 100–200 m bottom depth range the dominating species were the argentine (Argentina aliceae), deep-water seabasses (Hemanthias signifer and Pronotogrammus multifasciatus) and Pacific red snapper (Lutjanus peru). Notable in the catches were also lizardfish, widespur seabass and fortune jack (Seriola peruana). The mean density of fish recorded in this bottom depth stratum was considerably higher than in the shallower waters.

The shrimps found in deeper waters beyond about 150 m of depth were the nylon shrimp (Heterocarpus vicarius) and the colibri shrimp (Solenocera agassizii). Various penaeid shrimps inhabit the shallow depth range of which the crystal shrimps (Penaeus brevirostris), the western white shrimp (P. occidentalis) and the yellowleg shrimp (P. californiensis) were most frequent in the catches.

Trawl hauls were made on the well known ground for deep-water shrimp at 200–300 m on the slope off Punta Mala (Fig. 8.1) in order to test the availability of the shrimp there. The mean catch of nylon shrimp was about 500 kg/h, but with highest rates of several tonnes per hour. The rates were at a similar level as those obtained in previous exploratory surveys.

Of the two species of squids found, the dart squid (Loliolopsis diomedeae) was by far the most abundant while the Panama brief squid (Lolliguncula panamensis) was less frequently caught and only in minor amounts. These squids are semi-pelagic species aggregating near the bottom during the day, but dispersing into the water column at night.

The dart squid appeared to be distributed in special areas of high abundance at 50–100 m depth southeast and south of the Pearl Islands and in the southeastern part of the Gulf, where in February the mean catch rate was about 300 kg/h with a highest rate of about 1.5 t/h and in May the mean rate was about 500 kg/h and the highest 5 t/h. In the August and November surveys no catches were obtained demonstrating the strong seasonal fluctuations of this short-lived species.

The giant squid (Dosidicus gigas) was caught as incidental by-catch in 24 bottom trawl hauls on the slope with catch rates up to about 90 kg/h mostly from depths of 200–400 m and by both day and night. In the May survey a haul with the mid-water trawl gave a catch of approximately 0.5 t near the shelf edge.

8.3.2 Panama west coast, the Gulfs of Coiba and Chiriqui

The hydrographic environment on the western shelf is far more stable than that in the Gulf of Panama with only minor variations in the surface temperature and small changes in the depth and strength of the thermocline through the year.

Pelagic fish

Large parts of the western shelf were almost without any acoustic recordings of pelagic fish in all of the surveys. Aggregations of schooling pelagic fish were only located close inshore and the school areas were in most cases quite small. This inshore assemblage consisted mainly of the same species as in the Gulf of Panama: various species of anchovies, thread herring, different Carangidae and with barracudas as predators.

Demersal resources

The demersal fish fauna of this region can be grouped by three main habitats:

• the narrow shelf between Punta Mala and the Gulf of Coiba with a relative rich and varied fauna;

• the Gulfs of Coiba and Chiriqui with an extensive shelf but with poor demersal catches;

• the slope, from 100 to 400 m bottom depth. The slope was generally steep and narrow and was given only a limited coverage during the surveys.

The main demersal species on the narrow shelf between Punta Mala and Gulf of Coiba were various snappers (Lutjanus guttatus, L. peru, L. argentiventris, L. colorado), Peruvian mojarra (Diapterus peruvianus), barracuda (Sphyraena ensis), shark (Rhizoprionodon longurio) and bonefish (Albula vulpes). This fauna showed a marked difference both in size and composition, compared to the rest of the Panama shelf. This is probably related to the bottom type, sandy with patches of rocks and corals. The apparent absence of any trawl fishery may account for the relatively large size of the specimens.

The narrow shelf between Gulf of Panama and the Gulf of Coiba gave fairly good catches of commercially valuable fish. The snappers alone gave an average catch rate of 335 kg/h. The trawlable area is quite limited with patches and more extensive areas of rocks and corals. The area may, however, be suitable for a fishery with lines and traps. There were indications from the surveys that similar aggregations of demersal fish may be found further west along the Panama shelf. In the August survey good recordings were obtained over the Hannibal Bank, but in fishing trials with pots all gear was lost.

The main demersal species in the Gulfs of Coiba and Chiriqui were butterfishes (Peprilus medius and P. snyderi), lizardfishes (Synodus evermanni and S. scituliceps) and noncommercial species such as mojarra (Diapterus aureolus) and toadfish (Porichthys nautopaedium). Panama grunt (Pomadasys panamensis) and goatfish (Pseudupeneus grandisquamis) occurred in about one third of the catches but at low catch rates. The species composition in the two western Gulfs is quite similar to that of the Gulf of Panama.

The shrimps caught include the colibri shrimp (Solenocera agassizii) beyond 50 m bottom depth and the crystal shrimp (Penaeus brevirostris) in the 50–100 m bottom depth range. Various shrimps inhabit the 0–50 m bottom depth range of which the yellowleg shrimp (Penaeus californiensis) occurred in the catches most frequently.

Dart squid was found over wide parts of the western Gulfs and mostly in the 50–100 m depth range. Catch rates were considerably lower than in the Gulf of Panama.

Summary for Panama and biomass estimates

Small pelagic fish were found well inshore and above the thermocline. The highest densities were located along the northern, northeastern and western coasts of the Gulf of Panama. Only minor aggregations were recorded in the western Gulfs. The pelagic fish consisted of thread herring, anchovies, Carangidae mostly bumper with some lookdowns and jacks, while sierra and barracudas were present as predators. In addition to this inshore assemblage, pelagic fish occurred over the deeper offshore parts of the Gulf of Panama in May and August. These aggregations, which were almost completely absent in the February and November surveys, consisted of scad and round herring.

Demersal fish were found in highest densities in deeper waters, 50 m and more, and in the whole area the highest catch rates were obtained in the May-August surveys. Among the groups of potential commercial interest butterfishes dominated followed by grunts, snappers, sea basses and sharks. The fauna was similar in all gulfs, but an offshore component was located in the Gulf of Panama consisting of argentine, small sized sea basses and Pacific red snapper. High catch rates were obtained in one of the surveys in a limited area here. Relatively high densities of demersal fish, mainly snappers, some grunts, sea basses and sharks were found on the narrow shelf between Punta Mala and the Gulf of Coiba. There were indications that similar aggregations can be found further west, e.g., the Hannibal Bank.

The dart squid was found in special areas of high abundance in the 50–100 m depth range in the Gulf of Panama. It appeared to have an annual cycle of production. In the western Gulfs it occurred with far less abundance. Tests for giant squid confirmed its presence, but were inconclusive as fishing trials.

Some tests on the deep-water shrimp ground off Punta Mala confirmed that high catch rates of nylon shrimp can be obtained here.

Table 8.2 summarizes the biomass estimates for the Panama shelf by areas and types of resources. These are likely to lie below the true values for several groups and thus represent minimum levels. The total biomass exceeds 400,000 t and by far the main part, 88%, derives from the Gulf of Panama. The bulk of this again is from small pelagic fish, the inshore assemblage and the scads and round herring in the offshore part of the Gulf. Demersal fish and squid represent about 30% of the biomass in the Gulf. With a mean density of biomass of 44 t/nmi², (56 t/nmi² if an estimated biomass of 117,000 t of non-commercial bottom fish is included), the Gulf of Panama must be classified as an area of relatively high productivity. This is undoubtedly related to the local phenomenon of seasonal upwelling. With a mean density of 13 t/nmi² the western coast lies at a more typical level of tropical productivity.

Table 8.2 Panama: Estimated standing biomass by resource groups and sub-areas

8.4 COSTA RICA, 1987

The hydrographic environment over the Costa Rican shelf was fairly stable over the year with the thermocline reaching down to 70–80 m at which depth the oxygen content drops below 2 ml/l. Inside the Golfo Dulce this low oxygen level was reached already at 50 m of depth. The surface salinity was low in August and November probably as a result of increased river run-offs. In the westernmost part of the Costa Rican shelf, the Gulfs of Culebra and Papagayo, the environmental system is influenced by seasonal offshore winds causing upwelling.

Pelagic fish

Figure 8.4 shows as an example fish distribution in the February survey. Dense aggregations of pelagic schooling fish were located inshore especially in the Gulf of Nicoya, but often also eastward in the Bahia de Coronado. The species in this inshore assemblage were largely the same as in Panama, anchovies, clupeids, various Carangidae, barracuda and sierra, but the relative proportions seem to be different with more large-sized Carangidae and more of the predators including demersal fish. A deep-water assemblage was located in the border area between Costa Rica and Nicaragua with partly dense concentrations of hairtail (Trichiurus nitens), argentine (Argentina aliceae) and various seabasses.

Figure 8.4 Costa Rica: Pelagic fish distribution in the February survey

The dominating species of anchovy were Anchoa argentivittata, A. ischana and A. curta, and by far the most common clupeid was the yellowfin herring (Pliosteostoma lutipinnis).

The Carangidae contained a large number of species, but lookdowns, (Selene peruvianus and S. brevoorti) were most common, representing 50–60% of the catches. Various jacks, (species of Caranx, Hemicaranx, Carangoides and Oligoplites) represented together some 25% of the catches, while trevally (Gnathonodon speciosus) and pompanos (Trachinotus spp.) were less abundant.

Scombridae and the barracuda (Sphyraena ensis) had a relatively high availability, especially in the August survey. The sierra dominated, but with a few occasional catches of striped bonito (Sarda orientalis).

Demersal resources

The main demersal species in Costa Rica were snappers (Lutjanus guttatus and L. peru) and Peruvian mojarra (Diapterus peruvianus). Frequent in the catches, but at lower catch rates were butterfish (Peprilus spp.), toadfish (Porichthys nautopaedium), flounder (Cyclopsetta querna) and lizardfish (Synodus evermanni).

The mean catch rates of the species of commercial or potential commercial interest were low overall, about 110 kg/h, but the commercial fish in the catches were often large in size. The snappers were mostly caught inside the 50 m depth range. Good catches were obtained in the outer parts of Bahia de Coronado, in the entrance to Gulf of Nicoya and in locations close inshore from Cabo Blanco to Gulf of Culebra. Snappers usually prefer hard bottoms often unsuitable for trawling. This may cause underestimation when using trawl data for biomass estimates for this group. Some very large catches of small sized seabasses (Hemanthias signifer and Diplectrum macropoma), argentine (Argentina aliceae) and hairtails (Trichiurus nitens) were made at 100–200 m depth on the shelf edge in the border area with Nicaragua.

Deep-water nylon shrimp was found with langostino in the border area with Nicaragua.

Only few hauls were made for the deep-water nylon shrimp (Heterocarpus vicarius) as the slope was untrawlable over large parts except near the border with Nicaragua. Here a catch rate of about 500 kg/h was obtained for this species. Catch rates of colibri shrimp (Solenocera agassizii) at intermediate depths did not exceed 20 kg/h. Crystal shrimp (Penaeus brevirostris) appeared only sparingly, but no special night surveys were made for this species. Various shallow water shrimps appeared in the catches especially during the fourth survey in November.

Dart squid occurred in catches over most of the Costa Rican shelf. The catch rates were generally low.

Biomass estimates

Table 8.3 presents a summary of the biomass estimates which for several groups are thought to lie below the true levels. The total of 95,000 t which excludes the offshore assemblage in the north gives a density of 23 t/nmi².

Table 8.3 Costa Rica: Estimated standing biomass by resource groups

8.5 NICARAGUA (INCLUDING PARTS OF HONDURAS AND EL SALVADOR IN THE GULF OF FONSECA), 1987

North of Costa Rica, the coast and the shelf change character with a straighter coastline and a wider, even shelf. The slope, from about 200 to 500 m is smooth and fishable with bottom trawl over large parts. Prevailing offshore winds off the coast of southern Nicaragua during part of the year cause seasonal upwelling which undoubtedly contributes significantly to the productivity of the area.

The distribution of fish and crustaceans on the shelf from northwest Costa Rica to the Gulf of Tehuantepec in southern Mexico can be described in a simplified way by reference to inshore and offshore faunas. The inshore fauna was found along the coast out to a depth of 60–70 m and consisted of anchovies, Clupeidae (mainly thread herring), Carangidae such as bumper, lookdowns and jacks, sierra, barracuda, demersal fish, predominantly snappers and shallow-water shrimps and lobster. The offshore assemblage was found near the edge of the shelf and over the slope. Mesopelagic fish were recorded in many parts of the slope and further offshore and probably form an important component of the food chain of this community. This mainly consisted of argentine, hairtails, seabasses and cephalopods. Crustaceans dominated the upper slope areas, with langostino (Pleuroncodes planiceps), deep-water shrimp, especially nylon shrimp (Heterocarpus vicarius) and mantis shrimp (Squilla sp.) at greater depth. The pelagic giant squid (Dosidicus gigas) was mainly found off the shelf edge.

The intermediate shelf depths, from about 50–150 m seldom held any important quantities of fish, but in the upper part of this range, grounds of crystal shrimp (Penaeus brevirostris) could be found and shelf squids (Loliolopsis and Lolliguncula sp.) extended their depth range well below 100 m.

Pelagic fish

Pelagic fish was found in an inshore assemblage at depths down to about 50 m with smaller dense school areas usually between Corinto and San Juan del Sur. The stocks of small pelagics may be shared between Nicaragua and El Salvador. Their composition include anchovies, thread herring, Carangidae (mainly lookdown, bumper and jacks) and barracuda.

Offshore, an assemblage consisting of hairtails, argentine and seabasses was found near the shelf edge with the highest abundance off San Juan del Sur towards the border with Costa Rica. Very high catch rates were sometimes obtained.

Figure 8.5 shows as an example the distribution of fish over the shelf off Nicaragua, Honduras and El Salvador in the August-September survey. All surveys show similar features with a belt of recorded fish along the coast showing the inshore assemblage. Dense recordings of hairtails, argentines and seabasses were made offshore in the south in the February survey.

The Engraulidae consisted mainly of Anchoa spp., of which some seven were identified with A. argentivittata (silverstripe anchovy) and A. nasus as the most abundant. The most important clupeid species was the threadfin herring (Opisthonema libertate).

Catches of Carangidae included a number of species with lookdowns, almost exclusively Selene peruvianus and bumper (Chloroscombrus orqueta) followed by jacks of the genera Caranx, Carangoides, Hemicaranx and Oligoplites.

Figure 8.5 Distribution of fish over the shelf off Nicaragua, Honduras and El Salvador in the August-September survey

Sierras were relatively scarce throughout, but barracuda was common especially in August.

The mean catch rates in the bottom trawl of the pelagic fish inshore were generally low, up to a few hundred kg/h.

Very high catch rates, up to 80 t/h could be obtained of hairtails (Trichiurus nitens), argentine (Argentina aliceae) and seabasses, (Diplectrum and Hemanthias spp.) on the southern offshore grounds.

Demersal fish

Most of the demersal fish of assumed commercial interest were found within the 100 m depth range although the butterfish and the small seabasses were mostly caught at greater depths. The shallow demersal fish included snappers, grunts, and some sharks and croakers. Catch rates were generally low, but with a few high rates for snappers and croakers.

The main demersal species in the nearshore waters 0–50 m were Peruvian mojarra (Diapterus peruvianus), spotted rose snapper (Lutjanus guttatus), Panama grunt (Pomadasys panamensis) and threadfin (Polydactylus approximans). The intermediate zone (50–100 m) was dominated by searobin (Prionotus ruscarius), butterfish (Peprilus spp.), croaker (Cynoscion stolzmanni) and lizardfish (Synodus evermanni). The offshore region 100–200 m was characterized by argentine, rose threadfin bass (Hemanthias signifer), searobin, butterfish, cagua seabass (Diplectrum macropoma) and scorpionfish (Scorpaena spp.).

The average catch rate of commercial fish in all hauls with the bottom trawl in the 0–100 m range was low, about 90 kg/h with snappers dominating.

Crustaceans

Of the crustaceans, langostino was found in abundance in all surveys over large parts of the outer shelf and slope. Nylon shrimp gave occasionally high catch rates in the 200–300 m depth range. Crystal shrimp occurred at intermediate depth, but not in high densities.

The langostino (Pleuroncodes planiceps) was by far the most abundant crustacean appearing in high densities over a depth range from about 100 to 300 m. Areas of high abundance varied between the surveys but generally covered the outer shelf and the slope from Corinto southwards. In some parts of the slope, nylon shrimp (Heterocarpus vicarius) shared the deeper parts of the langostinos depth range and occasionally small amounts of colibri shrimp (Solenocera agassizii) the medium depths. Crystal shrimp (Penaeus brevirostris) was found at intermediate depths, especially in night hauls, but not with high catch rates. Various penaeid shrimps were taken as incidental by-catch in small quantities in the 15–50 m depth zone.

The catch rates of langostino were high with a mean of about 2 t/h in the 100–300 m depth range with a highest catch of 18 t/h. For nylon shrimp the corresponding rates in the 200–300 m depth zone were 400 kg/h and about 6 t/h. There was no difference between day and night catch rates for langostino, but night catches of nylon shrimp were only about 20% of the day rates.

Squid

Dart squid was found commonly in the 50–150 m depth range with some good catch rates. A modest effort in testing for giant squid with light and jigging confirmed the presence of this species and showed, in conformity with previous exploratory work, the highest catch rates off the western part of the slope.

The dart squid (Loliolopsis diomedeae) was caught over the mid-shelf (50–150 m) in all surveys with highest catch rates around 100 m depth. The species only occurred in bottom trawl during the day, lifting off the bottom at night. The mean catch rates in the 50–150 m range in the May and August surveys were about 100 kg/h with highest catches of 400 kg/h.

Incidental catches of giant squid (Dosidicus gigas) were taken in a number of bottom trawl hauls in the slope, but with low catch rates. More information of the distribution of this species was obtained from a modest programme of experiments with light attraction and jigging conducted with varying and never very high effort over the four surveys. The test programme demonstrated the presence of this resource and indicated that the highest availability seemed to be off western Nicaragua.

Biomass estimates for Nicaragua

Table 8.4 shows a summary of the assessments of the biomass of the standing stocks of the various groups. Some are likely to be underestimates, e.g., dart squid. The estimates for the offshore fish resources, especially the seabasses, are likely to have a low precision and further exploratory data should be obtained if there is an interest in this resource.

Table 8.4 Nicaragua: Summary of estimates of standing stock biomass

Since the weight of the shells form an unusually high proportion in langostino only part of it (perhaps some 25 %) should be included in evaluations of total productivity. This gives a total estimated biomass of 340,000 t for the Nicaraguan shelf, the mean density to 200 m depth is 50 t/nmi² and 62 t/nmi² if non-commercial bottom fish is included. This reflects a fairly high productivity which seems reasonable to relate to the process of seasonal upwelling identified in this region.

Gulf of Fonseca

All of the three countries Nicaragua, Honduras and El Salvador border on the shallow Gulf of Fonseca. It forms part of the environment of the inshore assemblage of fish and crustaceans and is an important shrimp fishing ground.

Particularly dense aggregations of fish were not recorded in the Gulf except in the November survey when a concentration of small pelagic fish, mainly anchovy, was located there.

The pelagic fish consisted of much the same species as in the neighbouring zones. The demersal fish classified as commercial consisted of croakers, catfishes and a few sharks and rays. The total mean catch rates in the bottom trawl were a few hundred kg/h.

8.6 EL SALVADOR, 1987

The shelf off El Salvador is wider near the Gulf of Fonseca and narrows towards the border with Guatemala. No important seasonal changes were detected, but there was a trend of a sharper and lifted thermocline in September-November as compared with March-May and this may have resulted in a more shoreward distribution of the assemblage above the thermocline in the autumn.

Pelagic fish

Pelagic fish was found inshore in all surveys with denser areas of schooling fish in patches westwards to Acajutla. The composition included anchovy, thread herring, Carangidae (mostly bumper, lookdowns and jacks) with barracuda as a fairly common predator. Fish was only sparsely recorded offshore, but occasionally high catch rates of argentine were obtained.

There was a continuous distribution inshore from Nicaragua through the Gulf of Fonseca westward along the coast of El Salvador (Fig. 8.5). The composition of the pelagic fish was similar to that further east, but here barracudas seem a more important predator than off Nicaragua.

On the offshore grounds the species were the same as those found in the offshore Nicaragua assemblage, hairtails, argentine and sea basses but the catch rates were considerably lower although argentine was still fairly abundant.

Demersal fish

The composition of demersal fish was similar to that observed in Nicaragua, but the densities were considerably lower, about one half to one quarter. The main demersal species in the nearshore waters 0–50 m were butterfish (Peprilus spp.), mojarra (Diapterus peruvianus), catfish (Bagre panamensis), Panama grunt (Pomadasys panamensis) and threadfin (Polydactylus approximans). Snappers were rare in the catches.

The intermediate zone (50–100 m) was dominated by searobin (Prionotus ruscarius), butterfish (Peprilus spp.), croaker (Micropogonias altipinnis) and mojarra.

The offshore region (100–200 m) was characterized by argentine, cagua seabass (Diplectrum macropoma), widespur seabass (D. euryplectrum) and scorpionfish (Pontinus sierra).

The deeper waters (200–300 m) were the main distribution area for langostino and mantis shrimp, and were inhabited by small quantities of hake (Merluccius angustimanus) and scorpionfish.

The mean catch rates of commercial species in the 0–200 m depth zone were very low, less than 100 kg/h.

Crustaceans

As in Nicaragua the most abundant crustacean resource was the langostino which had its main depth distribution between 150 and 300 m. The highest catch rates were taken somewhat deeper than in Nicaragua, the mean depth of hauls with more than 1 t/h was 220 m.

Nylon shrimp (Heterocarpus vicarius) and colibri shrimp (Solenocera agassizii) occurred in some hauls beyond 100 m. In the intermediate depths between 50 and 100 m, crystal shrimp (Penaeus brevirostris) was found, but generally at low catch rates. Various penaeid shrimps occurred in the 0–50 m bottom depth zone of which the whiteleg shrimp (Penaeus vannamei), the blue shrimp (P. stylirostris) and the Pacific seabob (Xiphopenaeus riveti) were the most common.

Both langostino and nylon shrimp were caught at high rates. More than 40% of the langostino catches exceed 1 t/h while about 25% of those for nylon shrimp exceeded 100 kg/h.

Squid

The distribution of dart squid at intermediate depth seemed to be continuous from the Nicaraguan up along the El Salvador shelf. Mean catch rates by surveys were highest in the May-June survey.

Jigging trials demonstrated the presence of giant squid. A few incidental catches of giant squid were taken in bottom trawl hauls in the southeastern part of the El Salvador slope area.

Biomass estimates for El Salvador

Table 8.5 summarizes the assessed stock biomasses of the important groups. Some of these, such as for dart squid and nylon shrimp are probably underestimates. If a quarter of the langostino biomass is included the mean density for the El Salvador shelf is 25 t/nmi².

Table 8.5 El Salvador: Summary of estimates of standing stock biomass

8.7 GUATEMALA, 1987

From the narrow part near the border to El Salvador the Guatemalan shelf widens off San José and forms a wide platform which continues into the Gulf of Tehuantepec off Mexico. As in El Salvador, the thermocline was sharper and shallower in late summer and autumn, September-December, than in winter-spring, March-June, but no important seasonal changes were observed.

Pelagic fish

Pelagic fish was found in an inshore zone to about 20 nmi from the coast, in quite extensive and dense school areas from San José to the Mexican border. Carangidae and Clupeidae dominated these aggregations. Surface schools of thread herring were observed inshore.

There was little variation in the distribution of pelagic fish in the four surveys along both the Guatemalan shelf and that of the Gulf of Tehuantepec in Mexico, and Figure 8.7 shows as an example that from the March survey. For management purposes the pelagic fish on the Guatemala-Gulf of Tehuantepec shelf may have to be considered as one shared unit stock.

Carangidae and Clupeidae dominated this inshore assemblage with some Engraulidae, Scombridae and barracudas.

Pacific thread herring (Opisthonema libertate) represented by far the main part of the clupeid catches with small bycatches of tropical longfin herring (Neoopisthopterus tropicus) and more occasionally yellowfin herring (Pliosteostoma lutipinnis).

Measured by the simple proportion in the total catches about 70% of the Carangidae were bumper (Chloroscombrus orqueta), about 20% lookdown (Selene spp.) and 10% consisted of other species mostly jacks of the genera Caranx, Carangoides, Hemicaranx and Oligoplites.

The fish catches in the trawl hauls over the deeper parts of the Guatemalan shelf were in general insignificant.

Figure 8.6 Guatemala and Gulf of Tehuantepec (Mexico): Distribution of pelagic fish in the March survey

Demersal fish

Demersal fish were dominated by grunts with some butterfish, snappers and sharks and small sized seabasses offshore.

Demersal fish was mostly restricted to depths less than 100 m. Inside 50 m the most common species were mojarra and grunt (Pomadasys axillaris) with some snapper (Lutjanus guttatus); within 100 m catfish (Bagre panamensis) and butterfish (Peprilus spp.) with some snapper (L. peru) and threadfin (Polydactylus approximans) were most common. The mean catch rate of commercial species was low, about 150 kg/h.

Crustaceans

Langostino was still a common species in the offshore parts of this shelf, with a mean catch rate of about 500 kg/h in the 100–300 m depth range. Deep-water shrimps were only caught occasionally. Crystal shrimp was found commonly at intermediate depths, but with low catch rates.

Squid

There were only minor indications of the presence of dart squid at intermediate depths on the shelf and of giant squid off the slope.

Biomass estimates for Guatemala

Table 8.6 summarizes the estimates of standing stock biomass for the various types of resources. With a mean density close to 50 t/nmi², the productivity on the Guatemalan shelf seems to be fairly high with small pelagics representing the main part.

Table 8.6 Guatemala: Summary of standing biomass by resource groups

8.8 THE GULF OF TEHUANTEPEC, MEXICO, 1987

The wide shelf in this area extends from the border with Guatemala along the coast up past Salina Cruz but becomes very narrow as the coast turns towards the southwest from this point. The outer part of the shelf forms a deeper platform at 200–300 m depths. The hydrographic environment has some special features, with the presence of pronounced oxycline at relatively shallow depths (50 to 100 m) and seasonal upwelling in late winter-spring.

Pelagic fish

Pelagic fish was found as a continuation of the distribution along the Guatemalan coast with some denser aggregations of schooling fish mostly east of about 94°. The thread herring was the most common species in this inshore assemblage with Carangidae dominated by bumper with some jacks and lookdowns and notable amounts of barracuda. Scads were found at medium depths in March and June when the thermocline deepened.

The possible environmental and/or biological role of the extensive lagoon systems should be considered. An addition to the inshore assemblage was recorded in March and June with scad (Decapterus macrosoma) at 50–100 m. This species was seasonally abundant in the offshore parts of the Gulf of Panama and occurred also in Costa Rica but was hardly noted in the intermediate region.

The only recordings made in the more distant offshore parts of the Gulf of Tehuantepec were of mesopelagic fish (Myctophidae). At times such recordings covered extensive parts of the deeper shelf.

The Carangidae were the most abundant group dominated by the bumper with some lookdowns and various jacks. The Clupeidae were mostly thread herring (Opisthonema libertate) with smaller catches of longfin herring (Neoopisthopterus tropicus) and yellowfin herring (Pliosteostoma lutipinnis). Barracudas were common in the catches especially in June and September.

The mean catch rates were some 400 kg/h.

Demersal resources

Demersal fish was dominated by grunts with some butterfish and snappers.

Demersal fish were mostly found inside the 100 m depth zone. In the 0–50 m range the most common species was mojarra and followed by snapper (Lutjanus peru), grunt (Orthopristis spp.), butterfish, catfish and bonefish. Common, but less abundant were lizardfish and threadfins (Polydactylus spp.). The mean catch rates of the commercial groups were low, about 100 kg/h.

Crustaceans

Crystal shrimp was found commonly at intermediate depths 50–100 m, with a mean catch rate (at night) of 19 kg/h.

Squid

Dart squid (Loliolopsis diomedeae) was caught at rates up to 120 kg/h in a limited area off Salina Cruz at intermediate depth (50–80 m) in March and December.

A few tests for giant squid with light attraction and jigging were made off the eastern slope in the March and June surveys. No catches were obtained.

Biomass estimates for the Gulf of Tehuantepec

Table 8.7 summarizes the estimates of standing stock biomass for the various types of resources. With a mean density close to 30 t/nmi², the productivity on the shelf in the Gulf of Tehuantepec was lower than in Guatemala. The high densities of mesopelagics observed in the Gulf of Tehuantepec are not included in this density estimate. Their abundance may relate to surface waters enriched by upwelling and advected offshore by the strong winds in the Gulf.

Table 8.7 Gulf of Tehuantepec, Mexico: Summary of estimates of standing standing stock biomass