Environmental Studies of the South Texas Outer Continental Shelf, 1977 Ichthyoplankton/Mackerel Eggs and Larvae

During the 1977 survey, 90,222 fish larvae and 59.,702 eggs were collected in the waters off the South Texas OCS. Both larval and egg numbers were higher than our 1976 catch which consisted of 75,082 larvae and 40,023 eggs. For the entire three year study from 1975-1977, 243,682 larvae and 156,651 eggs were collected and examined. Pelagic larvae representing 69 families, 117 genera and 80 species were identified in 1977. Larvae of the Spanish mackerel, king mackerel, bluefin tuna, cobia, mackerel, pompano and bluefish were documented for the first time during our studies in the northwestern Gulf of Mexico. The greatest number of species occurred in the myctophids (lanternfishes) followed by the sciaenids (drums) and scombrids (mackerels and tunas). The highest numbers of larvae were captured in May and the lowest in December. The highest numbers of eggs were also collected during May and the lowest during March and April. Family dominance varied on a seasonal basis. Our data indicate that no consistent larval or egg distribution was noted either inshore-offhsore or in the north south directions. Fish eggs were more abundant in water depths less than 45 m and were less abundant in the inshore waters. Spawning for many pelagic species probably occurred in water depths seaward of the continental shelf. Dragonets, lightfishes, mullets and tuna preferred the deeper offshore waters over the shelf while the herrings, anchovies and soles frequently spawned in water of intermediate depth. Generally, the spawning patterns, locations, and intensity varied between genera and species and between years. No specific spawning ground was noted for these fish. We were unable to obtain a series of mackerel eggs from known captive or wild stock so identification of these eggs was not possible below family level. During each year of this baseline survey, king mackerel larvae were captured in increasing numbers from May through September with at least 50 percent of the larvae being captured in September. King mackerel larvae were more abundant over the outer continental shelf (32 to 183 m). King mackerel larvae were taken in waters ranging from 20.2 to 29.8oC and were most abundant in waters ranging from 23.0 to 29.0oC. Spanish mackerel larvae were captured from May through September during each year of this study and were less abundant than king mackerel. Even though no sampling was done in October it is probable that both king and Spanish mackerel spawned during the same month. Few larvae were captured in either May 1975 or 1976 but they were more abundant in May 1977 than for any other month. The stations from 12 to 34 m yielded the highest concentrations of larvae. Spanish mackerel were more abundant during the day while king mackerel were more abundant at night. Spanish mackerel larvae were taken in waters ranging from 19.6 to 29.8oC and were more abundant in waters ranging from 20.2 to 29.8oC. During the three year period codlets were the highest in larval abundance during the winter of 1975 and 1976, while herrings were the most abundant in 1977. In the spring the anchovies were the most abundant during 1975 and 1976, while the lizardfish were the highest in 1977. The fall pattern showed the gobies the highest in 1975 and 1976 and the bothids in 1977. in comparing the Buccaneer Oil Field with BLM stations of comparable depth, no precise larval and egg abundance differences were noted between the areas. Direct comparisons of selected zooplankton taxa and ichthyoplankton indicated some linear correlations at the 95% and 99% confidence level which suggests that fish egg and larval abundance may be related to zooplankton abundance. In comparing bongo and neuston catches marked differences were noted in the catch of the two gears. Species diversity was also greater in the bongo tows when compared to the neuston tows. The effects of salinity and temperature on ichthyoplankton abundance showed little positive correlation than what normally would be expected for our sampling area. Changes in temperatures or salinity appeared to have no relationship to larval or egg mortality.