A comparative study of the life history: distribution and ecology of the sandbar shark and the gray reef shark in Hawaii

Abstract

The principle objective of this dissertation is to compare the ecology of 2 inshore Hawaiian sharks and to determine what factors are responsible for their local distribution. The sandbar shark, Carcharhinus milberti (Muller and Henle, 1841), is the most abundant shark around the high Hawaiian Islands but occurs nowhere else in the central Pacific. The gray reef shark, Carcharhinus menisorrah (Muller and Henle, 1841), is abundant in inshore areas throughout the central Pacific. In Hawaii, however, it seldom occurs near the main islands but is common around the smaller islands and islets. Around Oahu, adult sandbar sharks are significantly more abundant in the leeward areas between Diamond Head and Kaena Point (south and west coasts). Juveniles, on the other hand, were caught only in the windward areas. At Niihau, adults were caught in greatest numbers off the windward side (southeast coast) and no juveniles were caught. Gray reef sharks rarely occur around Oahu. At Niihau, catch rates for adults were greatest on the leeward side (north-west coast). Juveniles were caught only around the northern half of the island. Mature males of both species tend to occur in deeper water than the rest of the population. Females and juveniles occur at the shoalest depths of the species range with sub-adults at intermediate depths. The sex-maturity groups of the sandbar shark all occur at greater depths than those of the gray reef shark. The life histories of the 2 species are similar with respect to size at birth and maturity, fecundity and gestation period. Possible differences in growth rate and reproductive season are unlikely to influence local distribution. Distribution, however, was found to be correlated with physical factors of the environment. The sandbar shark occurs most commonly where currents are weak and where the bottom is level with a substratum of fine rubble, sand or mud. The gray reef shark is generally associated with clear water and hard bottom of rugged relief. These associations are more pronounced amongst juveniles than for adults and the above factors may be requisites for nursery areas. The diets of the sandbar shark and the gray reef shark are broadly overlapping. Both prefer fish with cephalopods ranking second. The sandbar shark tends to eat more crustaceans than the gray reef shark which may be related to its heavier tooth structure. The study species are preyed upon most heavily by the tiger shark and the galapagos shark. Predation does not appear to influence distribution though it probably plays a major role in limiting population size. It is concluded that differences in the distribution of the sandbar shark and the gray reef shark in Hawaii are probably a function of associations, especially among juveniles, with particular bottom types, turbidities and current velocities.