Chapter Ten: Demersal Fishes - Environmental Studies, South Texas Outer Continental Shelf, Biology and Chemistry
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A series of standardized, trawled fish samples were collected from six stations on each of four transects to provide information on South Texas Outer Continental Shelf (STOCS) fish distribution and abundance. Day and night collections were made at each each station during winter, late spring and late summer-autumn; supplementary day-night collections were made in intervening months at stations on Transect II. For each sample, numbers of individual species, weights, numbers of individuals per species, and individual weights and lengths provided the basic data, all of which except lengths and weights provided primary data for this study. For each sample, calculations of Shannon diversity indices (both numerical and ponderal), the Hurlbert probability of interspecific encounter (PIE) and the Lloyd and Ghelardi equitability value (E) provided derived data for various comparisons among the 1976 collections and between 1976 and the equivalent 1975 collections. For equivalent times and stations, the 1976 numbers of species, individuals, and biomasses were less than in 1975. Whether the slight change in sampling nets caused the declines or whether there were actually fewer fish in 1976 could not be demonstrated with completely tenable explanations. (A proposed analysis of length-weight-frequency analysis of individual species should clarify the viability of alternative explanations.) In general, both numerical and ponderal diversity indices were lower in 1976. Isopleth plots for day-night and seasonal data pertinent to observed and derived data indicated that there were pronounced day-night differences in most cases throughout the year. In the winter and spring, gradiewnts for the various data tended to be depth-related with some indication of north-south transect differentiation by autumn. Analysis of variance for the various data categories revealed that few individual effects (depths, transects, day-night, seasons) were consistently and statistically significant, but interactions involving seasons and individual effects were more. A pooled yearly comparison of day-night cataches by species indicated that day species were ordinarily those that had schooling propensities; predominantly nocturnal species tended to be solitary. Comparisons by the Wilcoxon rank sum showed statistically significant diurnal prominence for 10 species and nocturnal prominence for 36. Of the fishes not showing significant day-night prevalence in numbers or weights for pooled data, a breakdown of catches into seasons yielded statistically significant maxima in day-night differences in the spring and minima in the autumn. From published data, the day-night difference were related to activity and aggregational associations, to food habits, and to feeding tactics. The Bray-Curtis cluster analysis formulation was chosen over the Canberra-metric system for ascertaining station and species distributional characteristics. With species as attributes of the individual stations, the Bray-Curtis technique with flexible sorting by normal analysis showed clearly that there were depth related groupings, three in winter and four in spring and autumn. With stations as attributed of the species, inverse analysis showed seven species-groups in winter, eight in spring and six in autumn. The two-way relationships of station-groups and species-groups showed species-environmental relationships rather clearly for most of the species, 67 of 96 in winter, 68 of 89 in spring and 62 of 82 in autumn. In addition ot species associations determined by clustering, the cluster analyses indicated that zonation was depth-related, with temperature and seasonal migration patterns as major associative features of the groups through the seasons. There was little evidence that zonation was directly related to sediment type or salinity. The shallowest station groupings has high numbers of individuals, especially in winter and spring, and generally lower species diversities through the year. When temperatures were highest in late summer and autumn, nearshore species associations tended to dissipate; midwater and deepwater associations were somewhat more stable throughout the year; midshelf groups has the highest species diversity thoughout. There was a weak indication of species associations breaking into north-south groupings in autumn only, which implied that north or south movements to and from areas outside STOCS was relatively unimportant for the great majority of species. However, within the STOCS area there was considerable species shuffling during the year to the extent that clearcut species-domination by one a few species was not suggested. The 1975 and 1976 data, except for length-weight measurements of individual fish, are now incorporated into the data management system. Recommedations are suggested to: (a) further clarify relationships among the fish data inter alia, abd (b) relate the benthic fish data to other abiotic and biotic (especially invertebrate forage organisms) data, and (c) to explore selected population processes identified by the above analyses. Also recommended are acquisitions of data on feeding growth-metabolism rate properties of individual species. These recommendations are based on the premise that such functional characteristics and processes provide baselines that are immediately much more sensitive to environmental perturbations, both natural and man-induced, than are the generalized attributive inventories of raw counts and biomasses, changes in which can accrue only as long-term results of such functional characteristics and processes at the species and population levels.