Availability, morphometrics, feeding and breeding activity in a multi‐species, demersal fish stock of the Western Indian Ocean

Abstract

A multi‐species demersal fish stock was studied from April 1969 to July 1970. The common species are described in this paper and consist of Carcharhinus sealei, Rhynchobatus djeddensis, Dasyatis kuhlii, Rachycentron canadum, Psettodes erumei and Arius sp. With the exception of Rachycentron canadum, these fish were available throughout the year and some coincidence of increased availability was noted in August/September. Psettodes erumei, the commonest fish caught, was most available in August/September, February/ March and June. The size and weight distribution of each of these fish is described. Morphometric results indicate that two fish, Dasyatis kuhlii and Psettodes erumei, exhibit sexual dimorphism, the females being larger and heavier than the males. Carcharhinus sealei, Rhynchobatus djeddensis and Rachycentron canadum have a nearly isometric growth exponent whereas Dasyatis kuhlii, Psettodes erumei and Arius sp. have an allometric growth exponent. All fish were carnivores, Rhynchobatus djeddensis, Rachycentron canadum and Arius sp. being primarily crustacean feeders, Dasyatis kuhlii feeding on polychaetes and crustaceans and Carcharhinus sealei and Psettodes erumei feeding mainly on fish. The increased number of all three bony fish noted in August/September (immediately prior to the short rains in October/November) is discussed in relation to feeding activity and the initiation of spawning activity. It is suggested that spawning in Psettodes erumei and Arius sp. is initiated in September, continuing possibly to February and then during the long rains (March/May) and the South East Monsoon the fish recover. It is also suggested that pregnant elasmobranchs may come inshore in greater numbers during the rains to give birth.

Reproductive cycle, nutrition and growth of captive blue spotted stingray,Dasyatis kuhlii(Dasyatidae)

“…In a survey on a wild population of D. kuhlii in Tanzania of a total of 117 females caught throughout the year pregnant females were recorded in November (N ¼ 1), December (N ¼ 4), January (N ¼ 2), April (N ¼ 1) and July (N ¼ 1) (Darracott, 1977). This would indicate a breeding season.…”

“…Only one study was found to give the length (L)/weight (W) relationship of in situ D. kuhlii (Darracott, 1977). Of all animals (N ¼ 351) the relationship was expressed as W ¼ -2,73L 1,88 , for females (N ¼ 117) W ¼ -2,90L 2.0 and males (N ¼ 85) W ¼ -2.39L 1.65 .…”

“…at adult size (Ebert & Cowley, 2003). The diet of adult D. kuhlii in the wild consists of sand dwelling invertebrates (Lieske & Myers, 1994), like polychaete worms (Darracott, 1977) and crustaceans (Darracott, 1977;Compagno et al, 1989). Within this study the animals are fed a combination of teleosts (75%) and crustaceans (25%).…”

Dietary niche overlap in a nearshore elasmobranch mesopredator community

“…The number of trophic transfers between resource pools and elasmobranchs was estimated by calculating a standardized trophic level for each species with diet data as described by . For species lacking sufficient diet data, trophic level values were estimated from published diet information or from similar species (Darracott 1977, Stead & Bennett 2008, Schluessel et al 2010. Isotopic fractionation (δ 13 C: 0.96 ± 1.68 ‰; δ 15 N: 2.75 ± 1.64 ‰; Caut et al 2009) was scaled to the number of trophic transfers minus 1.…”

Morphological heterogeneity in some estuarine populations of the catfish <i>Arius jella</i> (Ariidae) in Sri Lanka

“…Accordingly, comparison of morphometric data among different groups should be preceded by a suitable correction to remove size-effect. Many Ariid catfishes show allometric growth (Darracott, 1977). All individual morphometric data were thus corrected, using the following equation (Elliott et al, 1995).…”