The Distribution, Biological Characteristics and Vulnerability of the Giant Sea Catfish, Arius thalassinus (Rüppell, 1837), to Fishing at Mafia Island, Tanzania

The distribution and some biological characteristics of commercially important giant sea catfish, Arius thalassinus (Ruppell, 1837) were studied at Mafia Island. Artisanal fishing catches were sampled, caught mainly with longlines, shark nets and ring nets. These yielded a total of 2 723 kg of A. thalassinus, comprising 756 individuals, the largest measuring 1 000 mm TL. Arius thalassinus occurred only on the western coast of Mafia Island and the highest catch rate was 19.3 kg.fisher-1.day-1 in March when murky water was predominant. The reproductive biology of A. thalassinus was investigated to assess its vulnerability to fishing. Arius thalassinus reached a size at first maturity (LM50) of 520 mm TL and exhibited a low mean (±SE) fecundity of 65.6 ± 3.37 eggs per female within the size range of 605-970 mm TL. The hydrated oocytes were large (mean diameter ±SE = 15.2±0.12 mm). Arius thalassinus spawned once in the study year during February and April, during heavy precipitation (124-499 mm). We therefore conclude that its restricted distribution, large size, low fecundity, late maturation and its reported high trophic level indicate that it would be vulnerable to fishing pressure. It is therefore recommended that fishing for A. thalassinus be restricted during its spawning season to ensure its sustainability. Corresponding author: ATK Email: kamukuru@udsm.ac. tz INTRODUCTION Catfishes (Order Siluriformes) are mostly freshwater fishes but two families (the Ariidae and Plotosidae) inhabit murky marine or brackish waters (Moyle & Cech Jr, 2004). These two families of sea catfish are regarded as unusual in that they live primarily in tropical and subtropical seawater, unlike the majority of catfish families that have little tolerance for brackish or marine conditions (Moyle & Cech Jr, 2004). The giant sea catfish, Arius thalassinus (Ruppell, 1837), is among 23 species of sea catfish that are mostly confined to murky coastal waters in the western Indian Ocean, within a depth range from 10 to 195 m (Fischer & Bianchi, 1984), and it is native to Tanzania (Bianchi, 1985). Studies on the distribution and biology of sea catfishes in Tanzanian coastal waters are still sparse. Bianchi (1985) recorded five species of the genus Arius (Family Ariidae) and two species of the genus Plotosus (Family Plotosidae) along the Tanzanian coast. All were found in shallow coastal waters, predominantly brackish, around major river mouths. Arius spp. are of high economic value and are caught using mainly hook and line, shore seines, shark nets and fixed traps (Fischer & Bianchi, 1984; Bianchi, 1985). Catch statistics have revealed the high value of ariid catfishes as they constitute 7% of the total fish caught in the Rufiji Delta, compared to Mafia Island where their contribution is significantly lower at 0.6% (MLFD, 2007). Ariid catfishes furthermore are carnivorous, feeding predominantly on crabs, prawns, mantis shrimps, fishes and molluscs (Taylor, 1986). They inhabit murky water, foraging by using one to four pairs of barbels (Moyle & Cech Jr., 2004), a characteristic of catfishes. Their ecological and reproductive characteristics include gonochorism, characterised by large-sized oocytes (Etchevers, 1978; Yanez-Arancibia & Lara-Dominguez, 1988), and low fecundity (Bruton, 1995), with the males being oral incubators (Shadashiv & Vivekanandan, 2008). Studies have confirmed that parental care is exhibited by A. thalassinus, as the fertilised eggs are carried in the buccal cavity of males for a month (Mojumder, 1978; Menon, 1991). Furthermore, giant sea catfish males refrain from feeding while carrying eggs, embryos or fry (Menon, 1991). O’Malley (2010) defined vulnerability as the intrinsic aspects in a species’ biology, such as its life history or ecological variables, which increases its sensitivity to or inhibits its recovery from an external threat. Morphological characteristics, life history and ecological traits such as area occupancy and rarity (Hawkins et al., 2000) and large body size, slow growth, late maturity, low reproductive output and high trophic level (Jennings et al., 1999), have been used to predict the vulnerability of marine fishes to fishing. The sex ratio in a species and evidence of parental care, spawning aggregations or sex change have also been considered predictors of vulnerability of marine fishes to fishing (Reynolds et al., 2001). Following the collapse of some fish stocks (Hutchings & Myers, 1996; Roberts & Hawkins, 1999; Hutchings & Reynolds 2004) and fishing threats to existing stocks (Sadovy & Cheung, 2003; Cheung, 2007), overfishing has remained the most pervasive threat to commercial marine fishes (Pitcher, 1998). Other threats to wild fish stocks include loss of essential habitats critical to complete their life cycle (Watling & Norse, 1998), climate change (Brander, 2010) and pollution (Sindermann, 1994). Currently, about 75% of wild fish stocks have been determined to be fully exploited (52%), overexploited (16%) or depleted (7%) (Botsford et al., 1997). Published information on the distribution and biological characteristics of A. thalassinus at Mafia Island is meagre. This study aimed to provide preliminary observations on the occurrence and some biological aspects of A. thalassinus at Mafia Island, especially those likely to predispose it to high fishing mortality.