Figure 4 illustrates the distribution of all fish as observed with the acoustic integration system. The unit of fish abundance is 0.1 m2 per nm2 acoustic reflection. With further analysis these observations can be converted to biomass by weight. Fish that occur down to about 0.5 m from the bottom will be observable with this system. The distribution chart shows three categories of fish concentration: 1-9 units = scattered fish; 10-49 units = slightly gathered fish; > 50 units = concentrated fish.
Guatemala
A distribution of fish was found inshore along the whole coast out to a depth of 40 - 50 m. Areas of relatively high abundance were found to the west of San Jose and towards the border with Mexico. The main part of this fish consisted of sardines, carangids, especially bumper and lookdown, barracudas and anchovies. In the offshore parts at depths of 40 - 50 m demersal fish, especially grunts and snappers occured together with scads. Parts of the anchovy and sardine schools consisted of juvenile fish. Areas of good sardine schools in mid water were located at shallow depths.
The small fish area indicated offshore in Figure 4 is silver smelt located at abt 180 m of depth.
Golfo de Tehuantepec, Mexico.
Figure 4 shows that the aggregation of fish found inshore along the Guatemalan coast continues along the Tehuantepec Gulf coast and beyond Salina Cruz. The areas of highest density are located close to the shore, generally at depths less than 20 m. The fish occured in schools during the day and in looser schools and layers at night. The most common species were sardines, various scads especially bumper and lookdown, barracudas, Spanish mackerel, butterfish, which also seemed to occur in schools, and grunts. Towards Salina Cruz sardine became less abundant, but the abundance of demersal fish which are less observable by acoustic systems increased. The fish distribution in this part extends further away from the shore, out to about 60 m of bottom depth.
Over the offshore parts of the shelf only mesopelagic fish (Myctophids) were recorded. These were found in layers near the surface at night and in schools and layers at 200 - 300 m depth during the day. Very dense recordings of mesopelagic fish were made over some of the deeper parts of the shelf (not included in Figure 4) and with their adaptation to oxygen -deficient conditions they probably form an important part of the biological production and the food chain in this system.
