Simple algebraic relationships and yield equations that require the minimum of data are developed so as to enable quick and reliable assessments of relative rate of harvesting tropical freshwater fish populations. The age of a fish at the inflexion point is inversely related to the growth rate (K) and directly related to the natural logarithm of the weight length exponent (b). Algebraic relationships between the exponent of anabolism (m) and the weight length exponent are developed. Equations for estimating total mortality from age and length distributions in catch samples are given. Total mortality for both continuous and discrete recruitment are considered. The probability density function and the discrete probability function for a negative exponential are given. The effect of a number of variables on trawl catches is studied and some multiple regression equations which might be used to assess the relative degree of exploitation are presented. The parameters which have been recognized as vital to yield prediction are: the growth rate (K), the weight length exponent (b), the maximum length to which a fish grows (L∞) and the natural and total mortality rates. It is shown that one can replace age with a length expression in yield models and still have reliable yield predictions. It is also shown that a model with a few very relevant parameters, has almost the same predictive power as a model requiring more parameters. / Science, Faculty of / Zoology, Department of / Graduate

This study investigates two aspects of tropical reef fish mating systems: the prevalence of paternal care in those species that exhibit some form of parental care, and the evolutionary stability of simultaneous hermaphroditism. Studies of three species of tropical reef fishes from the Gulf of California were used to investigate these two different types of mating systems. Two species of demersal egg guarders with paternal care, Malacoctenus hubbsi and Abudefduf troschelii, were studied to determine the importance of a male's mating status (the number of eggs in his territory) in regard to female choice of mates. In both species, females did not base their choice of mates on the number of eggs in the male's territory, males increased their level of parental care with increasing egg number, and egg survivorship where estimated was independent of brood size. These results corroborate, and offer the first experimental field verification of a crucial assumption of a minimal-care hypothesis proposed a decade ago by Williams (1975). Male reproductive success was studied in greater detail in one species, M. hubbsi. Territory parameters, rather than male size, were the major determinants of male reproductive success. This independence relative to male size was attributed to the unpredictability of good future territories. In Serranus fasciatus all individuals begin reproduction as simultaneous hermaphrodites, but lose female function at large sizes to become males. Males obtain disproportionate reproductive success by maintaining harems of hermaphrodites and defending them from adjacent males. These harems are similar to those found in some sequentially hermaphroditic species. Hermaphrodites obtain most of their reproductive success through female function by spawning with a male, but also engage in an alternative male strategy--streaking on pair spawns--to obtain some male reproductive success. This small amount of male reproductive success by hermaphrodites appears to stabilize simultaneous hermaphroditism with most resources allocated to female function. Facultative alternative male mating strategies in hermaphroditic individuals joins spawning reciprocation as the only known behavioral strategies that provide evolutionary stability for simultaneous hermaphroditic individuals in populations of tropical reef fishes.