3.9 Sta. Marta to Gulf of Morrosquillo

3.9.1 Small pelagic fish
3.9.2 Demersal resources
3.9.3 Overview and discussion of survey results

The easternmost part of the shelf west of Sta. Marta from Gulf of Morrosquillo to Gulf of Uraba was only covered in the first survey as bottom conditions were found to be unsuitable for trawling in the latter Gulf.

3.9.1 Small pelagic fish

As can be seen from Figure 3.8.2 only few and restricted aggregations of fish were recorded west of Sta. Marta in any of the surveys. Clupeids, mainly thread herring occurred only in the catches in any amounts in the first survey. The carangids had a more consistent distribution with mean catch rates for surveys 1, 2 and 3 of 36, 14 and 2 kg/hour and highest rates of 384, 137 and 11 respectively. Bumper, bigeye scad and Caranx species dominated. Scombrids, barracudas, sharks and squid appeared with very low rates only.

Estimates of biomass from the acoustic density observations were 16 000, 6 000 and 17 000 tonnes for surveys 1 through 3. This gives a mean available standing stock of small pelagic fish of 13 000 tonnes.

3.9.2 Demersal resources

Demersal fish

The shelf from Sta. Marta and westwards was covered with a trawl-survey input of 9, 15 and 15 stations in surveys 1 through 3. The trawl data have been analysed by two bottom depth strata 0-50 m and 50-120 m, but does not reveal systematic difference in species composition by depth.

Catch composition

Table 3.9.1 shows the mean catch rates in kg/hour and the catch distribution by size classes for the main species of demersal fish. The four dominating species are vermilion snapper Rhomboplites aurorubens, lane snapper Lutjanus synagris, mutton snapper L. analis and southern red snapper L. purpureus. Silver mojarra is also common in the catches, but has a lesser commercial value.

Table 3.9.1 Colombia west. Mean catch rates and catch distribution by size classes for all successful swept-area hauls carried out.

Species		Mean rate	Number of hauls in catch groups.						Rate of occurrence*
		kg/hour	1-30 kg	30-100 kg	0.1-03 t	0.3-1 t	1-3 t	>3 t
INNER SHELF 0-50 m
	Vermillion snapper	8	26	1	1				28/38
	Mutton snapper	6	12	2					14/38
	Silver mojarra	6	20	1					21/38
	Lane snapper	5	26	1					27/38
OUTER SHELF 50-120m
	Lane snapper	10
	Vermillion snapper	9
	Mutton snapper	7
	Silver mojarra	7

* calculated on basis of all hauls on shelf.

Table 3.9.2 Colombia west. Mean catch rates (kg/hour) in all hauls by main commercial groups.

Family	INNER SHELF	OUTER SHELF	ALL SHELF
	0-50m	50-120m	0-120m
Snappers	23	30	26
Groupers	1	1	1
Croakers	1	1	1
Grunts	1	0	0
Seabreams	0	0	0
Other demersal	23	19	21

Biomass estimates

Biomass estimates by main species groups are shown in Table 3.9.3 and are obtained by multiplying the estimated densities, shown in Annex 7 of the DATA FILE, with the area of the shelf, calculated by planimeter to 2 100 nm for the 0-50 m depth zone, and 1 800 nm² for the 50-100 m zone.

The table shows that the snappers are the only notable resource of demersal fish, estimated to 3 400 tonnes when data from the three surveys are pooled. Other commercial resources, such as croakers, groupers and seabreams sum up to only 340 tonnes, and other demersal, mainly of lesser commercial value, to 2 700 tonnes.

Table 3.9.3 Colombia west. Estimates of biomass of demersal fish by main groups and surveys.

		Survey 1	Survey 2	Survey 3	All surveys
INNER PART 0-50 m
	Snappers	250	1 400	2 700	1 600
	Croakers	200	0	20	60
	Grunts	0	200	40	60
	Groupers	0	130	60	80
	Seabreams	0	0	40	20
	Other demersal	800	1 100	2 500	1 600
OUTER PART >50 m
	Snappers	4 600	1 500	1 100	1 800
	Croakers	0	100	0	50
	Grunts	20	0	0	0
	Groupers	0	50	20	50
	Seabreams	0	20	0	20
	Other demersal	1 000	1 600	500	1 100
	Total, main groups	5 100	3 400	4 000	3 700
	Total, other dem.	1 800	2 700	3 000	2 700

3.9.3 Overview and discussion of survey results

The following resources were identified and will be discussed:

The acoustic survey showed that aggregations of small pelagic fish consisting of sardinella, thread herring and carangids could be found with medium densities and over restricted areas mainly along the shelf from Sta. Marta northeastwards 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 possibly be related to the season of most intensive upwelling. There appears to be a continuous distribution of the pelagic fish from the shelf north of the Guajira Peninsula into the outer part of the Gulf of Venezuela. Very little pelagic fish was found along the shelf between Sta. Marta and the Gulf of Uraba.

The random trawl survey showed that snappers represented the most important demersal group while grunts, groupers, croakers and seabreams were at much lower densities. An analysis by depth strata did not reveal any 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 joint stocks is thus suggested.

Shelf squids Loligo pealei and L. plei occurred with relatively high catch rates in a zone around the Guajira Peninsula in the March survey, but lower in the other surveys probably reflecting an annual production cycle.

Table 3.9.4 shows the biomass estimates by groups and areas. A calculation of mean biomass density per unit shelf area to a depth of 100 m gives 23 t/nm² for the Colombian Atlantic coast, which indicates an overall low production. This measure calculated for the shelf northwest of Sta. Marta has, however, a level of 48 t/nm² indicating a fairly good level of productivity. The corresponding estimate for the southwest shelf is 5 t/nm² which demonstrate the paucity of resources here.

The expectation to resource availability in this area has centered on the shelf off the Guajira Peninsula and the upwelling process here as a source of primary production. The results confirm that the major part of the resources on the Colombian Atlantic shelf are indeed located in the region off the Peninsula. The stock biomasses of the species which in the first instance make use of the upwelling production, sardinella and thread herring are, however, not very large. But this accords with the description of the upwelling process under chapter 2.1 Hydrography above, as weak and not very persistent. The surveys have thus confirmed the existence of special conditions for fish production off the Guajira Peninsula, but one may perhaps conclude that the significance of the phenomenon is limited.

Various previous resource surveys have been made of this area. Under the UNDP/FAO Fisheries Project, 1968-1972 (Anon. 1973) a bottom trawl survey was made of the inner shelf which in coverage would correspond roughly to that of our work. The estimated biomass of species of commercial interest was 65 000 tonnes. This includes pelagic fish caught in the bottom trawl and represents the same general level of resource availability as that found in our survey.

Some comments may be made regarding the sustained yield that may be obtained from these levels of standing stock. If it is assumed that the resources are moderately exploited at present, the yield may be assessed as 20% of the biomass for the more long-lived demersal fish and 30% for the squid and the small pelagic fish. This gives total potentials including present catches of about 3 000 tonnes demersal fish, mainly snappers, 1500 tonnes of shelf squids and 30-40 000 tonnes of pelagic fish. Whether any fishery can be developed on the generally low valued resources of small pelagic fish is a question which must be carefully considered, among other things taking into account the distribution in time and space and fish behaviour as related to catchability.

It must be noted that the surveys did not cover the shallow inshore waters. The important resources of shrimp on the shelf and the tuna resources which is likely to be associated with the upwelling system off the Guajira Peninsula are also excluded. The total assemblage of resources is thus somewhat underestimated.

Table 3.9.4 Colombia. Overview of estimates of biomasses by main groups. Tonnes.

		Guajira Pen. to Sta. Marta	Shelf west of Sta. Marta	Total
Pelagic fish
	Sardinella	60 000		60 000
	Thread herring	40 000		40 000
	Carangids	14 000		14 000
	Scombr. etc.	4 000		4 000
	Total	118 000	13 000	131 000
Demersal fish
	Snappers	6 700	3 400	10 100
	Croakers	550	110	660
	Groupers	620	130	750
	Grunts	1 450	60	1 510
	Seabreams	700	50	750
	Total commerc.	10 020	3 750	13 770
	Others	4 200	2 700	6 900
	Total demers.	14 220	6 450	20 670
Squid		5 000		5 000
Total		137 220	19 450	156 670