Survey objectives and effort
Within the framework of the UNDP/FAO Project GLO/82/001 arrangements were made for a survey programme with the R/V DR. FRIDTJOF NANSEN during 1988 of the shelf region of the northern coast of South America from Suriname to the western border of Colombia.
The programme was planned at a meeting between representatives of the coastal countries of the region, IMR, Bergen, FAO and ORSTOM (Office de la Recherche Scientifique et Technique d'Outre Mer) called by FAO in Port of Spain, Trinidad in January 1988. The main part of the planned work was successfully completed through four surveys during 1988. Cruise reports with outlines of the work done and some main findings were issued after each survey in four parts (IMR, 1988a to 1988p) and a final report (Strømme and Sætersdal, 1989a and b) was presented at a seminar in Cartagena, Colombia in October 1989. This chapter provides a brief review of the surveys and the findings.
The general objectives were, to:
describe the composition, distribution and abundance of small pelagic and demersal fish and crustaceans on the shelf and slope, to evaluate the potentials of the stocks;
collect biological samples of the most important species for studies of growth, maturity, etc.;
conduct taxonomic studies for the purpose of preparing a regional species guide;
conduct oceanographic investigations.
The main survey methods were echo integration with trawl sampling for pelagic fish and bottom trawling for demersal resources. In areas where snappers occurred in mid-water over hard bottom, sampling with hand lines was to be tried. Fishing in shallow-water shrimp areas was not expected to cover the stocks for biomass estimates, but trawl results would provide biological samples and distributional characteristics.
Test hauls would be made to provide information on catch rates and species composition of deep-water shrimp on the slope at 200–900 m from Suriname westwards.
The one year programme January-December 1988 allowed for four surveys spaced about equally through the year. Generally the shelf from about 20 m to the edge was investigated with work also on the slope when testing for deep-water shrimp and squid. Table 9.1 shows some details of the investigational effort, which did not vary much between surveys and the averages are shown. The acoustic degree of coverage was adequate throughout, but highest on the Venezuelan shelf because of the higher abundance of small pelagics in this area.
Table 9.1 Details of investigational effort for the four surveys: months and the average (of four surveys) of duration, survey distance, degree of coverage and number of trawl stations per sub-area
|Sub-area||Months||Duration (days)||Distance (nmi)||Degree of coverage||Trawl stations|
|Suriname||Jan May Aug Nov||5.4||1,060||8.3||38|
|Guyana||Feb May Aug Nov||5.1||813||6.5||42|
|Venezuela Orinoco||Feb May Aug Nov||2.6||458||6.1||20|
|Joint fishing area||Feb May Aug Nov||1.5||200||5.7||25|
|Trinidad||Feb May Aug Nov||4.0||800||10.7||22|
|Venezuela oriente||Feb Jun Sep Nov||8.0||1,390||12.6||58|
|Venezuela west||Feb Jun Sep Dec||10.8||1,290||17.4||61|
|Colombia||Mar Jun Sep Dec||6.5||900||10.0||41|
The total field effort of the survey comprised about 170 days of active research work with some 27,000 nmi steaming and 1,200 trawl stations.
The distribution of the survey effort did not vary much between the surveys. Figure 9.1 shows as an example the course tracks and fishing stations in the October-November survey for the shelf from Suriname to Orinoco. A similar survey coverage was made of the shelf regions of Trinidad, the east and west coast of Venezuela and Colombia.
Figure 9.1 Course tracks and stations Suriname to Orinoco, October-November 1988
Environmental and faunistic studies
The results of the hydrographic investigations of the surveys described on the background of the existing knowledge of the meteorology and oceanography of the region was presented in the final report (Strømme and Sætersdal, 1989a and b).
Briefly the hydrography can be described by three coastal regions:
Region 1: Suriname, Guyana and eastern Trinidad and Tobago;
Region 2: Trinidad and Tobago to Golfo de Venezuela;
Region 3: Peninsula de Guajira to Golfo de Uraba.
Both from meteorological and oceanographic points of view the surveyed area maintains a rather stable climate on a short time scale basis, however subjected to a fairly strong seasonal signal. This is due to the stability of the trade winds which blow steadily throughout three-quarters of the year.
The dominant feature of the oceanography of the coastal regions is the proximity of a major ocean current system: in the eastern areas the Guiana Current, which is an extention of the South Equatorial Current and in the northern areas (Venezuela and Colombia) the Carribbean Current, which in the coastal zone predominantly is an extention of the Guiana Current.
The ocean current system, flowing northward with a coast on its left side, indicates its presence by sloping density surfaces upwards towards the coast in both the Atlantic and Caribbean regions. This feature is present at all times of the year, but is subject to regional and seasonal disturbances.
The sloping isopycnals on the Guiana coast do not demonstrate true upwelling; it is more a stationary feature, although responsible for the elevation of colder water, usually rich in nutrients, to higher levels in the sea. Combined with wind-induced vertical eddy exchange, the structure will complete the transport of nutrients to the euphotic zone. The discharge of large amounts of freshwater on the Guiana shelf is also a phenomenon of significance for fishery oceanography.
The deflection of the coastal water of the Caribbean to the north results in a prolonged and intensive upwelling along the eastern Venezuelan coast. Because of the Caribbean Current being further offshore and the configuration of the coast, there is no upwelling off western Venezuela.
Off the Guajira Peninsula only a modest seasonal upwelling was detected, but the surface layers were stable along the Colombian coast to the southwest.
The type of bottom on the shelf and on the slope was observed acoustically along the survey tracks. Based on examination of the echograms, four categories were distinguished:
Charts of these observations were presented in the final report (Strømme and Sætersdal, 1989a)
Work started by FAO taxonomists in these surveys contributed to the later preparation of a species guide for fishery purposes (Cervigón et al., 1993), while also the assemblages of demersal fish in the region were analysed and described (Strømme and Sætersdal, 1989a and Bianchi, 1992a).
General trends in the distribution of pelagic fish off Suriname were similar in the four surveys, Figure 9.2 shows the echo integration charts from the January-February survey. This shows that pelagic fish were found aggregated in areas over the inner shelf where they occurred in schools and layers. Fish densities were high and the areas extensive in all except the third survey in August. This assemblage consisted of a large number of species from three main families, Engraulidae, Clupeidae and Carangidae accompanied by larger-sized predators, barracudas, Scombridae and sharks. Lower densities of pelagic fish were found over the outer shelf consisting of fewer species, mainly round sardinella and rough scad and other Carangidae. Pelagic fish also formed an important component of the catches of the bottom trawl.
Various Anchoa species dominated the Engraulidae, while the most common genera of the Clupeidae were Opisthonema, Pellona and Chirocentrodon. Bumper (Chloroscombrus chrysurus), lookdowns (Selene spp.), and horse mackerel (Trachurus lathami) were the most abundant Carangidae. The Scombridae were mainly the king mackerel (Scomberomorus cavalla) with some sierra (S. brasiliensis) and the barracudas, mainly Sphyraena guachancho, with less S. picudilla.
Figure 9.2 Suriname-Orinoco: Distribution of pelagic fish January-February 1988. (Car: Carangidae; Clu: Clupeidae; Eng: Engraulidae; Lut: Lutjanidae; Scm: Scombridae; Sph: Sphyraenidae; Tri: Trichiuridae)
The main demersal species groups found in Suriname waters were snappers, croakers and grunts. The outer shelf was clearly dominated by snappers, while the the inner shelf in addition held important amounts of croakers and grunts.
On the outer shelf the main species were vermillion snapper (Rhomboplites aurorubens), southern red snapper (L. purpureus) and cardinal snapper (Pristipomoides macrophthalmus).
The main species on the inner shelf were: lane snapper (Lutjanus synagris), Corocoro grunt (Orthopristis ruber), king weakfish (Macrodon ancylodon), Acoupa weakfish (Cynoscion acoupa), dwarf goatfish (Upeneus parvus), American harvestfish (Peprilus paru) and Jamaica weakfish (Cynoscion jamaicensis) in decreasing order of abundance.
The demersal fish distribution extended continuously from Suriname into the shelf of Guyana. The stocks seem thus to be shared, but to what extent is uncertain.
The mean catch rates for all surveys were:
|Inner shelf 0–50 m|
|Outer shelf 50–120 m|
Occurrence of sharks in the catches seemed to be associated with that of their main prey, small pelagic fish, but catch rates were modest and the incidence low.
Only incidental observations were made on the important resources of shallow-water shrimps. Fishing tests were made for deep-sea shrimps.
Squids were found in modest amounts in the mid-shelf consisting mainly of small Loligo species. The highest catch rates were obtained in the May survey.
Summary of biomass estimates
Table 9.2 summarizes the assessments of the standing stock of the various groups. Some are likely to have been underestimates, e.g., sharks and perhaps red snapper. With a total biomass of 693,500 t (excluding shrimps) and a shelf area of 15,000 nmi2 the mean density of biomass of the Suriname shelf was 45 nmi2 which indicates a fairly high productivity.
The swept-area biomass estimate of demersal fish including larger pelagics Scombridae, barracudas and the semi-demersal hairtails was 150,000 t of which 128,000 t on the inner shelf (0–50 m)
Summarized data are available on the findings from a number of previous trawl surveys of the shelf of Suriname. In order to compare the various estimates of standing biomass the same catchability coefficient (q) must be applied in the calculations. In Table 9.3 where the results for each survey have been converted to the use of q = 1, a considerable variation can be noted. Some of the variability could be attributable to differences in survey methodology, e.g. surveys targeting on shrimp may have a high proportion of night hauls which will have a negative bias on fish catches. Also the groups of fish included may have varied between surveys. The comparison may, however, demonstrate a real decline of demersal fish biomass on the outer shelf over the long period from the 1960s, when these offshore grounds must have held accumulated virgin stocks till to date. On the inner shelf, however, the DR. FRIDTJOF NANSEN results fell reasonably well into the ranges of the previous findings, but here a careful examination of the species composition is required.
Table 9.2 Suriname: Summary of estimates of standing stock biomass
|Resource group||Biomass t||%|
|Other demersal mostly non-commercial||46,500||37|
Table 9.3 Suriname: Comparison of estimates of standing biomass (t) from various trawl surveys with calculations adjusted to a catchability coefficient q = 1
|Vessel||Year||Inner shelf||Outer shelf||Total|
|DR. FRIDTJOF NANSEN||1988||128,000||22,000||150,000|
|Sources: Klima, 1976 and Losse, 1982.|
Pelagic fish were found mainly in dense inshore aggregations which extended from the border with Suriname onto the southeast Guyana shelf. West of the Essequibo River densities were low. The catch composition by groups was roughly the same as in Suriname: Clupeidae, Engraulidae and Carangidae with some large pelagic predators, but with lower mean catch rates.
The principal groups were the same as in Suriname: snappers, croakers and grunts.
The main species on the inner shelf were in order of importance and using the pooled data from all surveys: king weakfish (Macrodon ancylodon), southern red snapper (Lutjanus purpureus), green weakfish (Cynoscion virescens) and shortfin corvina (Isopisthus parvipinnis). On the outer shelf the dominating species were vermillion snapper (Rhomboplites aurorubens), cardinal snapper (Pristipomoides macrophthalmus) and Acoupa weakfish (Cynoscion acoupa). As mentioned above according to their distributional pattern these stocks must to some extent be considered as shared with Suriname.
Only incidental observations were made of the important resources of shallow-water shrimps.
Squids were found in modest amounts in the mid-shelf consisting mainly of small Loligo spp. some times with a patchy distribution.
Summary of biomass estimates
Table 9.4 shows a summary of the assessments of the standing stock of the various groups. Some of these are likely to be especially negatively biased such as those for sharks and perhaps the red snapper. With a total biomass of 374,200 t and a shelf area of 13,900 nmi2 the mean density of biomass of the shelf of Guyana was 27 t/nmi2 which indicates a moderate productivity. The total biomass may be somewhat underestimated, but the density is markedly lower than that found off Suriname. A part of the difference could be explained by a higher rate of exploitation in Guyana.
The swept-area trawl survey showed a total biomass of 87,000 t, which included, demersals, Scombridae, barracudas, and hairtails.
Data are available on the findings from previous trawl surveys of the Guyana shelf. In Table 9.5 the results for each survey have been converted to the use of the same catchability coefficient as that applied to the results of the DR. FRIDTJOF NANSEN surveys, viz., q = 1. There is a considerable similarity between the standing stock estimates for the inner shelf. The low standing stock found in the DR. FRIDTJOF NANSEN survey on the outer part of the Guyana shelf as compared to the results of the OREGON II 1972–77 may demonstrate a stock decline brought about by fishing.
Table 9.4 Guyana: Summary of estimates of standing stock biomass
|Resource group||Biomass t||%|
|Shallow water shrimp|
Table 9.5 Guyana: Comparison of estimates of standing biomass (t) from various surveys with calculations adjusted to a catchability coefficient q = 1
|Vessel||Years||Inner shelf||Outer shelf||Total|
|DR. FRIDTJOF NANSEN||1988||76,500||10,700||87,000|
|Source: Klima, 1976 and Fabres, 1980|
Various parts of the shelf off the coasts of Trinidad are affected by different ecological regimes:
the Colombus Channel in the south can be considered an extention of the Orinoco estuary and is dominated by the seasonal discharges of this large river. Under an agreement with Venezuela this part is defined as the “Joint Fishing Zone south of Trinidad and north of Venezuela”;
the east coast, where the Orinoco regime extends along the inner parts of the shelf, while its more offshore parts are covered by the Guiana Current;
the shelf off the north coast is located at the start of the Caribbean Current with its inshore upwelling system and relatively high productivity;
the Gulf of Paria, which was not surveyed.
The pelagic and demersal resources of the north coast and the Joint Fishing Zone were surveyed with the standard methods, and the east coast with echo integration only and some test hauls for deep-water shrimp. Rough bottom prevented a coverage of the outer shelf of the north and east coasts by the trawl survey for demersal fish.
It should also be recalled that this type of survey does not adequately cover large pelagic species such as Spanish mackerel. The description of the resources will therefore be somewhat inadequate. However, the main features of the distribution of the resources are probably reflected in the data and the estimates of standing stocks can be used for comparison with other sources of information.
The small pelagic fish were in general found to be limited to the inshore parts of the shelf and the low densities offshore probably give a true picture of the distribution of these resources.
Table 9.6 summarizes the biomass estimates for the different parts of the coast by main groups. A minimum estimate of the biomass density within 100 m of depth is 26 t/nmi2 which indicates a moderate level of productivity. A further breakdown of the 38,000 t biomass of Carangidae, etc., by using the proportions of the total catch indicates a biomass of hairtails, barracudas and Scombridae of 15,000 t. A swept-area estimate of these species referring to the north coast and the Columbus Channel only, gives 4,000 t which for these mainly pelagic species must be a clear underestimate. The best assessment of the standing stock of this group based on the data is probably 10,000 t.
Table 9.6 Trinidad and Joint Fishing Zone: Overview of estimates of biomasses by main groups (t)
|North coast||East coast||Joint Fishing Zone||Total|
|Anchovies & Sardinellas||6,000||16,000||24,000||46,000|
|* No estimate available|
The Venezuelan coast falls naturally into four regions with environmental regimes which differ significantly and affect the composition and abundance of the fish resources:
the Orinoco shelf between Guyana and Trinidad and the eastern part of the north coast;
the so-called Oriente which covers the shelf from the Dragon's Mouth to Cabo Codera;
the western part of the main coastline to the Paraguana peninsula;
the Gulf of Venezuela, in part shared with Colombia.
The Oriente with its broad shelf and favourable conditions for high production through upwelling represents the richest and most interesting part, whereas the west coast has a narrow shelf and a generally low productivity and the Gulf of Venezuela the characteristics of a shallow enclosed sea area.
This small shelf between Guyana and Trinidad is characterized by a soft muddy bottom prevalent over the inner and mid-shelf and an inshore hydrography dominated by the discharges of the Orinoco River.
Small pelagic fish were found in aggregations in a narrow inshore belt with largely moderate densities where catch compositions included anchovies, sardines, and Carangidae, with some hairtails and Spanish mackerel.
Demersal fish was found mainly on the inner shelf with croakers as the main group and giving high catch rates. The distribution of croakers extended also out to the mid-shelf region where the main species was cardinal snapper (Pristipomoides macrophthalmus).
Sharks were caught in modest amounts mostly at depths less than 40 m.
Estimates of the standing stocks were 70,000 t of pelagic fish and 30,000 t of demersals. There was a special uncertainty concerning the estimates on this shelf because of the large inshore shallow areas which could not be covered.
In the analysis of the fishing experiments a distinction was made between the data from the inner part of the shelf, inside the Testigos Archipelago and Margarita Island and the coastal side of the Cariaco Trench and those from the outer part, outside the Islands and westwards past Tortuga island. The main difference in the fish assemblages of these two parts was that the inner shelf was dominated by small pelagic fish, sardines and anchovies with catfish and croakers as the most common demersal fish, while the pelagic fish of outer part was characterized by horse mackerel and mackerel and as demersal groups snappers and grunts with some breams, groupers and glasseyes. Hauls were made at various depths along the outer slope to test the availability of deep-sea shrimps and cephalopods.
In all surveys pelagic fish were found in abundance on the inner shelf along the coast from the Gulf of Cariaco eastwards. The main component was the Venezuelan sardine (Sardinella aurita).
Figure 9.3 shows the distribution of pelagic fish in the four surveys from the echo integration data.
The geographical distribution of the school areas of sardinella conformed with previous finding from sardine surveys of the Oriente and with the general information from the fishery. High densities were observed in a narrow inshore zone from the Dragons Mouth westwards past Cabo Tres Puntas towards Isla Margarita, in the bay outside Peninsula de Araya and in the Golfo de Cariaco, especially its western part and the entrance. A pattern of seasonal migration along the coast may be indicated with an easternmost location in November, some shift westwards in February, a westernmost distribution in May-June and the start of a shift towards the east in August-September. This could be related to a spawning and reproduction cycle which includes a westwards drift of larvae and a larval-and early juvenile stage adjusted to the season of high rates of upwelling and production.
There was a clear relationship between high-density school areas and inshore cool upwelled water.
Estimates of the biomass (Table 9.7) were made for two pelagic groups: Pelagic 1 consisting of Clupeidae and Engraulidae and Pelagic 2 consisting mainly of Carangidae, especially rough scad (Trachurus lathami), Scombridae, barracudas and hairtails.
Table 9.7 Oriente: Estimates of standing biomass (t) of pelagic fish by surveys
|Survey||Pelagic 1||Pelagic 2||Total|
Figure 9.3 Venezuela, Oriente: Distribution of pelagic fish, mainly Venezuelan sardine in the four surveys. (Anch: anchovies; Car: Carangidae; Dem: demersal fish; Sard: Sardinella; Scom: Scombridae)
The variation of the estimates between surveys is unlikely to reflect stock variations for the Pelagic 2 group, more probably changes in availability. The mean standing stocks were estimated as follows:
|Venezuelan sardine||800,000 t|
|Other Clupeidae and anchovies||200,000 t|
|Rough scad||180,000 t|
|Other Carangidae||20,000 t|
|Scombridae and barracudas||20,000 t|
The main demersal fish on the inner shelf were catfish, croakers and grunts, with lesser amounts of groupers. The outer shelf was clearly dominated by snappers and grunts but contained also minor amounts of groupers, croakers and seabreams.
The principal species for the inner shelf were: catfishes (Cathorops spixii and Bagre marinus), barbel drum (Ctenosciaena gracilcirrhus), bronze-striped grunt (Haemulon boschmae), cardinal snapper (Pristipomoides macrophthalmus) and whitemouth croaker (Micropogonias furnieri). For the outer shelf the main species were: cardinal snapper, bronze striped grunt, vermillion snapper (Rhomboplites aurorubens), dwarf goatfish (Upeneus parvus) and Atlantic bigeye (Priacanthus arenatus).
Biomass estimates for the main commercial bottom fishes varied between 27,000 and 55,000, with a mean of 47,000 t.
Small-sized squids (Loligo spp.) were common over large parts of the shelf with the highest abundance in the intermediate depths near Margarita Island. There was a clear seasonal trend in the catches with highest rates in the February and May-June surveys, undoubtedly reflecting the production cycle for these short-lived species. Swept-area biomass estimates for two first surveys were: 7,000 t and 8,200 t respectively. The catches consisted nearly exclusively of the two species, Loligo plei, (70% on average) and L. pealei (30%).
Test hauls were made for deep-sea shrimp at 200–800 m depth in the fishable parts of the slope between the Testigos Archipelago and Tortuga Island. The catch rates were low with means of 5–10 kg/h for the main species, with the highest rates up to 50 kg/h. The megalops shrimp (Penaeopsis serrata) was the most common species at 300–400 m and the royal red shrimp (Pleoticus robustus) at 300–500 m. The giant red shrimp (Aristaemorpha folicea) and the striped red shrimp (Aristeus varidens) occurred at intermediate depths, 400–500 m, while the scarlet shrimp (Plesiopenaeus edwardsianus) was taken in a few hauls at 700–800 m. The Caribbean lobster (Metanephrops binghami) was caught in a few hauls at 300 m depth.
The shelf along the West coast of Venezuela from Cabo Codera to the Paraguana Peninsula is narrow. the biological productivity of these inshore coastal waters is known to be low and this was confirmed by the findings of the surveys.
The pelagic fish were dominated by Carangidae, the most abundant species of which was the rough scad (Trachurus lathami). The mean biomass estimates were:
|Clupeidae and anchovies||10,000 t|
|Scombridae and barracudas||8,000 t|
The demersal fish fauna was mainly composed of snappers. The main species were mutton snapper (Lutjanus analis), lane snapper (L. synagris), vermillion snapper (Rhomboplites aurorubens) and cardinal snapper (Pristipomoides macrophthalmus). The mean biomass estimate of the commercial bottom fishes which included croakers, seabreams and grunts was 8,500 t.
In some test hauls for deep-sea shrimp at depths between 200 and 400 m only low catch rates were obtained.
This enclosed sea proved to support fish stocks of only moderate abundance.
The schooling behaviour of pelagic fish was different from that found in the Oriente with numerous smaller schools distributed in patches over the various parts of the Gulf. Patches of high densities were not extensive and mainly restricted to the outer part of the Gulf.
The pelagic fish biomass were assessed at:
|Clupeidae and anchovies||100,000 t|
The demersal fish fauna was mainly composed of croakers inside the Gulf and snappers in the deeper waters of the mouth. The dominating species were whitemouth croaker (Micropogonias furnieri), American harvestfish (Peprilus parus) and lane snapper (Lutjanus synagris) in the inner part of the Gulf and Atlantic bigeye (Priacanthus arenatus), mutton snapper (L. analis) and dwarf goatfish (Upeneus parvus) in the deeper waters in the mouth. The inner shallow waters of the Gulf were insufficiently covered and the total biomass estimate for the Gulf of about 10,000 t may have been too low especially for the croakers.
Squids, mainly Loligo pealei and L. plei, showed a seasonal cycle with the highest abundance in February and June with a swept-area estimate of 2,500 t.
The most notable finding was the high abundance of fish found on the Oriente shelf from the Dragons Mouth to Cabo Codera in all four surveys. The narrow shelf further west to the Paraguana Peninsula was found to contain only very sparce resources. In the Gulf of Venezuela small pelagics appeared seasonally in variable abundance and demersal fish had moderate abundance.
It should be noted that some areas of importance for small-scale fisheries could not be covered by the surveys, viz. the inner shallow parts of the Gulf of Venezuela and Lake of Maracaibo.
The biomass estimates are reviewed in Table 9.8.
Table 9.8 Venezuela: Review of biomass estimates (t)
Using the shelf area to a depth of 200 m, the total standing stock per unit shelf area was as follows: for the Oriente 109 t/nmi2 which indicates a high productivity, for the West Coast 10 t/nmi2 a very low figure and for the Gulf of Venezuela 20 t/nmi2, which indicates a modest level of production.
Among the previous investigations of Venezuela's marine fishery resources, the programmes conducted by Fundación La Salle de Ciencias Naturales in co-operation with ORSTOM are most directly comparable with the DR. FRIDTJOF NANSEN programme. Reporting on results of acoustic surveys during 1980–85 under this programme Gerlotto and Gines (1988) presented assessments of total pelagic fish biomass for the Oriente region of 1,400,000 t of which sardinella was estimated to represent between 730,000 and 1,000,000 t. This compares well with the assessments presented in Table 9.8 of respectively 1,220,000 and 800,000 t.
Expectations as to resource availability on the Atlantic coast of Colombia had been centred on the shelf off the Guajira Peninsula and the upwelling processes nearby as a source of primary production. During the surveys it appeared, however, that the upwelling was rather local and not very pronounced, strongest in the winter, and weak in the summer and autumn observations.
The acoustic survey showed that aggregations of small pelagic fish consisting of sardinella, thread herring and Carangidae could be found with medium densities and over restricted areas mainly along the shelf from Santa Marta northeastward and especially to the northwest and north of the Guajira Peninsula. Sardinella dominated in the March and June surveys and thread herring in the September and December surveys. This may be related to the season of most intensive upwelling. There appears to be a continuous distribution of pelagic fish from the shelf north of the Guajira Peninsula into the outer part of the Gulf of Venezuela and the resources in this area may thus be shared between Colombia and Venezuela. Very little pelagic fish was found along the shelf between Santa Marta and the Uraba Gulf.
The most important pelagic species were sardinella (Sardinella aurita), thread herring (Opisthonema oglinum), rough scad (Trachurus lathami) and various other Carangidae.
The trawl survey showed that snappers represented the most important demersal group while grunts, groupers, croakers and seabreams were found at much lower densities. An analysis by depth strata revealed no consistent difference in species composition by depth. Catch rates on the southwestern shelf were much lower than in the northeast, but snappers dominated the catches also here. A significant increase in the densities of snappers off the Guajira Peninsula in the December survey coincided with observed decreased rates in the Gulf of Venezuela in the same survey and a possible connection between the two areas with shared stocks is thus suggested.
The main demersal species were lane snapper (Lutjanus synagris), vermillion snapper (Rhomboplites aurorubens), mutton snapper (Lutjanus analis), southern red snapper (L. purpureus) and tomtate grunt (Haemulon aurolineatum), while triggerfish (Balistes capriscus) was also frequent in the catches.
Squids (Loligo pealei and L. plei) occurred with relatively high catch rates in a zone around the Guajira Peninsula in the March survey, but with lower rates in the other surveys probably reflecting an annual production cycle.
A few hauls were made at 300–500 m depth west of Santa Marta to test the presence of deep-sea shrimps. Giant red shrimp (Aristaemorpha folicea) and royal red shrimp (Pleoticus robustus) were among the species which occurred in modest amounts, with catches up to about 2 kg/h. These data added no information additional to that provided by the very extensive surveys made jointly with the Japanese Agency for International Co-operation (JICA) in 1980.
The biomass estimates by main groups are shown for the two parts of the coast in Table 9.9.
Table 9.9 Colombia: Biomass estimates by main groups (t)
|Guajira Peninsula- Santa Marta||Shelf west of Santa Marta||Total|
|Sub-total all demersal||14,000||6,000||20,000|
A calculation of mean biomass density per unit shelf area to a depth of 100 m gives 23 t/nmi2 for the Colombian Atlantic coast, which indicates low production. However, the shelf northwest of Santa Marta has a of standing stock biomass level of 48 t/nmi2 indicating a fairly good level of productivity. The corresponding estimate for the southwest shelf is 5 t/nmi2 which demonstrate the paucity of resources here.
It must be noted that the DR. FRIDTJOF NANSEN surveys did not cover the shallow inshore waters. The important resources of shrimp on the shelf and the tuna resources which are supposed to be associated with the upwelling system off the Guajira Peninsula were not covered. The total assemblage of resources was thus somewhat underestimated by the surveys.
Various resource surveys have been made of this area. Under a UNDP/FAO Fisheries Project, 1968–72, a bottom trawl survey was made of the inner shelf which in coverage that corresponded roughly to that of the DR. FRIDTJOF NANSEN survey. The estimated biomass of species of commercial interest was 65,000 t which included pelagic fish caught in the bottom trawl. This represents a similar general level of resource availability as found in the DR. FRIDTJOF NANSEN surveys.