Trap Fishery and Reproductive Biology of the Whitespotted Rabbitfish Siganus sutor ( Siganidae ) , within the Dar es Salaam Marine Reserves , Tanzania

Fish samples and catch data were collected monthly from traditional basket fish traps set on reefs within the Dar es Salaam Marine Reserves (DMRs) to evaluate trap catch dynamics and reproductive biology of the rabbit fish Siganus sutor (Valenciennes 1835). Trap catch composition was 85% rabbitfishes by weight with a mean (± SE) catch per unit effort (CPUE) of 2.8 ± 0.1 kg trap-1 day-1 during a fishing season, which lasted between September 2004 and May 2005. CPUE did not differ significantly with lunar cycle, but exhibited a spatial variation, the highest being from Fungu Yasini Marine Reserve, which is further from shore. The spawning season of S. sutor is protracted, lasting between October and May, peaking in March. Size at first sexual maturity was 217.1 and 227.7 mm TL for male and female S. sutor, respectively. The species exhibits a size-dependent sex ratio, with a preponderance of females at larger sizes. The species shows high fecundity at a maximum of 1,300,400 oocytes for 396 mm TL female. Average (± SE) total fecundity was 627,052 ± 78,886 oocytes per female of 266 to 396 mm TL. It is proposed that beach seines be eliminated from the marine reserves and regulated trap fishery be allowed as a way to facilitate sustainable exploitation of the rabbitfishes and other reef fishes in the DMRs.


INTRODUCTION
Traditional trap fishing is one of the main sources of income and animal protein for coastal people in East Africa (King, 2000).Yields from the shallow reef fisheries have been declining mostly as a result of overfishing caused by poor fishing methods and increased effort (McClanahan & Mangi, 2001;Kamukuru et al., 2004Kamukuru et al., , 2005)).It is widely recognised that fishing leads to changes in the abundance of target species manifested by decreases in sizes and catch per unit effort (CPUE) within the fishery (Koslow et al., 1988;Alcala & Russ, 1990;Kamukuru et al., 2005).Global fisheries statistics show that after depleting large and slow-growing fishes at higher food webs, fisheries increasingly rely on smaller and fast-growing fishes at lower trophic levels (Sibert et al., 2006).This, "fishing down the marine food web" (see Pauly et al., 2005) presents exploitation patterns that are unsustainable.
In East Africa, hexagonal basket fish traps (madema) woven from wood and split bamboo are commonly used by artisanal fishers.McClanahan & Mangi (2004) describe two basket trap sizes, as either small (1.2 × 1 × 0.2 m) or big (2 × 1.3 × 0.3 m), in terms of length, width and height with a mesh size of about 3 or 5 cm, respectively.Fishing techniques entail placement of baited traps (using an assortment of green algae and animal tissues) on the reef flat, in sandy lagoon areas interspersed with seagrasses and coral rubble to target reef fishes.In order to maintain position and appear as a refuge the traps are covered by broken fragments of rock and coral.They are usually hauled on a daily basis, but sometimes are left in water for more than a day depending on prevailing weather conditions.
Wild siganids are an important human food source with reported catch of 480 tonnes in 1981 in the Western Indian Ocean region (Fischer & Bianchi, 1984).Their fast growth rate (Kaunda-Arara & Rose, 2006), diurnal schooling behaviour and browsing in shallow water habitats make them ideal for trap fishery and aquaculture (Woodland, 1990).
Some data on the seasonal reproduction of the whitespotted rabbitfish, Siganus sutor, are available for the Kenyan coast (Ntiba & Jaccarini, 1990, 1992).However, Bwathondi (1981) reported that siganids breed throughout the year in Tanzanian coastal waters.There is limited information on the fishery of S. sutor, the most commercial siganid, from Eastern Africa and most of the WIO region.The present study investigated variations in CPUE and size structure of S. sutor from trap fishery within the Dar es Salaam Marine Reserves and provides additional information on reproductive parameters of the species.

Study area
Dar es Salaam Marine Reserves (DMRs) refer to a set of three island reserves of Bongoyo, Mbudya and Pangavini and, Fungu Yasini sand bank (Fig. 1).DMRs were gazetted in June 1975, however, little effort has been made to conserve their habitats and ensure sustainable resource use (URT, 2005).DMRs are shallow reef areas (less than 20 m deep) in which four sites namely Mbudya, Pangavini, Fungu Yasini and Fungu Mkaji were studied between September 2004 and May 2005.Study sites were chosen because they were considered typical coral reef areas targeted by migrant trap fishers from Unguja Island in Zanzibar.

Data collection
Fish caught in basket traps of mesh size of 5 cm set overnight in <15 m depth were weighed on a suspended spring balance to the nearest 0.5 kg.Catch was sorted into rabbitfishes and non-targeted fish categories.Catch data were recorded over 12 days for each lunar month (i.e. three consecutive days for each new moon, full moon, first quarter and last quarter) for the entire fishing season.Information on the number of fishers per boat, traps per boat and fishing grounds were provided by fishers.Individuals of S. sutor were also collected from beach seines of mesh size 1.5 cm and 4 cm at the bunt and wings respectively frequently used by fishers in weedy shallow waters adjacent to DMRs.Data from the two fishing gear were used to compare length-frequency distributions and size at first capture of S. sutor.
S. sutor was measured for total length (TL) on a fish measuring-board to the nearest millimetre.Total weight (TW) and eviscerated weight (EW) were measured to the nearest gram using a toploading digital balance.Paired gonads were weighed to the nearest 0.1 g.Macroscopic gonadal maturation staging followed the key by Ntiba & Jaccarini (1990).Paired ovaries at maturity stages IV and V were preserved in cross-referenced plastic bottles containing Gilson's fluid for fecundity determination.Contents of the bottles were kept at room temperature for three months with a constant vigorous shaking to allow release of eggs from ovarian tissues.Presence or absence of viscera fats (VF) surrounding the gonads and gut was recorded for every individual fish examined.Gonadosomatic indices (GSI) were determined as: Total fecundity (F) of each female was obtained by counting standing stock of advanced yolked oocytes in ripening ovaries using a volumetric method (Kamukuru & Mgaya 2004) as: where V is volume to which the total number of yolked oocytes was made up, n is the mean number of yolked oocytes in ten aliquots, and v is the volume of sub-sample.The length at which 50% of the fish were mature (Stage III and above) was regarded as the size at first sexual maturity (L M50 ).The L M50 was estimated by fitting the fraction of mature individuals per 10 mm TL intervals to an where %MF is the percentage of mature fish by size class, L is the mid length of each size class, a and L M50 are constants for the model.Size at first capture was determined using the method by Pauly (1984).In addition, seawater temperature was recorded using a temperature logger placed at 2 m depth, recording daily temperature over 4.8 hours per day.

Data analyses
Average number of fishing boats, fishers, traps, CPUE and fish lengths were compared among months, sites and lunar phases and fishing areas using single factor ANOVA and post-hoc test followed the Turkey honestly significant difference (HSD) test.Normality of the data was checked with the Shapiro-Wilkinson test and homogeneity of variances was determined with a Levene's test using SPSS 15.0 for Windows (SPSS Inc., 2006).All data that did not show homogenous variances were log (x + 1) transformed before analysis.Sex ratio of S. sutor was analysed using the Chi-square test.The least squares regression analysis was used to investigate the relationships between fish length and fecundity and between length and weight.

DISCUSSION
Rabbitfishes constituted 85% by weight of fish caught in basket traps within DMRs.Roxburgh et al. (2002) reported rabbitfishes to constitute up to 32% by weight from all of the fishing gear in the study area.These results suggest that the DMRs have a high abundance of Siganidae.The impact of the Asian tsunami of December 26, 2004 on the Tanzanian coast likely explains a dramatic decrease A progressive decline in CPUE could partly be attributed to a sharp decline in fish lengths in May towards the end of fishing season, a phenomenon reported elsewhere by Koslow et al. (1988) and Alcala & Russ (1990).These studies show that fishing affects fish size structure and that when fish sizes are smaller CPUE declines as fish weight increases exponentially with fish length.Unavailability of suitable bait due to rough seas coinciding with the onset of SE monsoon season might have contributed to the cease of basket trap fishery in DMRs (fishers pers.com.).Basket trap off fishing season (June -August) is normally characterised by lower water temperature, high wind energy, strong East African Coastal Current (EACC) and low solar insolation (McClanahan, 1988) that might have posed difficult conditions for the basket trap fishery.
Higher CPUE and large-sized S. sutor were observed from Fungu Yasini and Mbudya Island Marine Reserves.This may likely be attributed to spatial variations in fishing effort as a result of the distance separating fishing communities and marine reserves boundaries.For example, Fungu Yasini Marine Reserve, separated from the shore by a 5 km wide channel, had a significantly higher CPUE and large-sized S. sutor compared to Pangavini Island Marine Reserve situated at 1 km from the shore.It is also likely that frequent beach seining in the easily accessible areas adjacent to Pangavini Island might have reduced the abundance and size structure of S. sutor.Lugendo et al. (2005) indicated that shallow weedy habitats are frequently utilised by juveniles of commercially important fish species in marine embayment areas in Tanzania.Lunar cycle had no significant influence on the catchability of rabbitfishes by basket traps.Therefore, rabbitfishes either enter traps during daytime looking for food (i.e.bait) or at night seeking for shelter.Froese et al. (2003) described rabbitfishes as being diurnal herbivores, hiding in reef crevices during the night and browsing over reefs during the daytime.
The study demonstrated the effect of fishing gear on the size of target fish with beach seines predominantly catching sub-adult fish (107.3 ± 1.3 mm TL) compared to basket traps (262.0 ± 1.8 mm TL).Seine nets in Tulear region, southwest Madagascar, also target small rabbitfishes of average sizes <100 mm that consumers did not favour (Laroche & Ramananarivo, 1995).Beach seines were also found predominantly catching subadult fish in the Kenyan marine artisanal fisheries (McClanahan & Mangi (2004).For a sustainable S. sutor fishery the gear should retain desirable sizes that do not undermine the sustainability of fish stocks.In this study, S. sutor size at first capture using basket traps was greater than the size at first sexual maturity (see Fig. 6) thus allowing fish to reproduce before they are caught.
S. sutor showed a skewed size dependent sex ratio with females dominating at larger size classes.Similar observations are reported for S. rivulatus and S. luridus, on the Lebanese coast (Bariche et al., 2003).Another study has suggested a higher growth rate of female S. sutor than male S. sutor (Ntiba & Jaccarini, 1988).Ecologically, it makes sense for females to delay attaining sexual maturity because of significant positive relationship between body dimensions and egg production.Furthermore, Wootton (1998) stated that the cost of egg production is greater than that for sperm, thus requiring females to start reproducing when they have attained a larger size than males.
The occurrence of ripening and fully ripe fish between October and May, and peak GSI in March accompanied by a gradual decrease in the occurrence of VF were consistent with an extended spawning period of S. sutor.The depletion of body fat reserves in several fishes has been observed during gonadal maturation (Wootton, 1998).It is likely that oocyte recrudescence in S. sutor is related to high sea surface temperature extending from October to a maximum of 29.4º C in April (see Fig. 5b).Different spawning seasons of rabbitfishes in tropical and sub-tropical waters have been reported (Ntiba & Jaccarini, 1990;Al-Ghais, 1993;Yeldan & Avşar, 2000;Bariche et al., 2003).Takemura et al. (2004) suggested that reproductive seasonality of rabbitfishes appears to be affected by regional variation in environmental factors such as seawater temperature, photoperiod, and food abundance.Seawater temperature has been found to play a role in the timing and duration of spawning season of siganids in the Mediterranean Sea (Takemura et al., 2004).
DMRs are no-take zones but fishing has not been banned and the most damaging fishing practice is the prevalent use of beach seines in the shallow adjacent areas.Reef-fish resource conservation based on MPAs with weak enforcement may be ill-suited for protection of species that are highly susceptible to overfishing such as reef fishes.This study has demonstrated the potential of basket trap fishery in DMRs as a method for sustainable exploitation of the siganids.One of the management options in DMRs could be a ban of beach seines causing growth overfishing and restricting S. sutor trap fishery during peak spawning season likely to cause recruitment overfishing.It is recommended that long term catch and effort data collection combined with fish size measurements be established in DMRs in order to allow estimates of maximum sustainable yield (MSY), a useful tool for fisheries management.

Fig. 2 .
Fig. 2. Monthly variations in a) number of fishing boats, traps per boat and fishers per boat and b) CPUE in the Dar es Salaam Marine Reserves.Bars are standard errors (SE) of the mean

Fig. 3 .Fig. 5 .Fig. 6 .
Fig. 3. (a) Spatial and (b) lunar cycle variations of mean CPUE for basket trap fishery within the Dar es Salaam Marine Reserves.Bars are standard errors (SE) of the mean