FISHERIES RESOURCES DIVISION

NIGERIAN INSTITUTE FOR OCEANOGRAPHY AND MARINE RESEARCH

P.M.B. 12729, VICTORIA ISLAND, LAGOS.

Wet and dry season variations in shrimps and bycatch compositions were surveyed in the artisanal beam trawl shrimp fisheries operating in near shore water (<20m depth and <2 nautical miles) in South West Nigeria. Significant variabilities in targets shrimp Nematopalaemon hastatus and bycatch compositions were detected at these temporal scales; shrimps – bycatch ratio of 14:1 and 6:1 were obtained respectively at both wet and dry seasons. The total quantity of shrimps and bycatch landings combined were more in wet season (9749.22kg) than in dry season (4090.71kg), T-test comparison was significant (P<0,05). Twenty five (25) species belonging to 20 families of fishes constitutes the bycatch species and are juveniles with a total length range of 4 to 30cm. While majority of the bycatch species occurs throughout the year, few have seasonal occurrence; Arius latiscutatus, Eretmochellys, Imbrica, Callinectes amnicola in wet season and Sardinella aurita, Chloroscumbrus chrysurus and Pseudotolithus typus in dry season. The results are discussed in terms of; (i) The rational for the introduction of management options such as seasonal closures or otherwise technological changes in gear design to sustain the coastal fishery resources, (ii) The contrast shrimp – bycatch ratio in this fishery.

Artisanal beam trawl shrimping in Nigeria is a daily fishery and is carried out throughout the year. The fishery is new and operate using an improvised planked canoe (LOA 8-9.5 m) powered by 25 or 40 HP outboard engine in estuaries and nearshore part of the sea within 1-2 nautical miles from shoreline where their technology could permit. These zones are reserved for small-scale fisheries exploitation by the Nigerian fisheries law and regulations and its form the nursery ground for juveniles of finfishes. The incidental capture and mortality of these fishes that when larger are targeted in other commercial inshore and offshore Nigerian fisheries is considered irresponsible, unsustainable and capable of reducing the future yield of other fishery.

In 1994, bycatch from shrimp trawl fisheries was estimated to be around 11.2 million tones worldwide (Alverson et al, 1994). A global awareness of bycatch problems has led to various management strategies that attempt to alleviate some of the impacts of large bycatches (Andrew and Pepperel, 1992). One such option is to restrict trawling to locations and times or season known to have relatively small amount of bycatches (High et al, 1969; Caddy, 1982). The present work was therefore to find out whether this shrimp fisheries is characterized by wet and dry season variations in bycatch species compositions, shrimp – bycatch ratio and shrimp – bycatch quantity for the purpose of recommending pragmatic management strategy to alleviate bycatch problems.

The assessment survey was conducted in two years (August 2001 to July 2003). A total of 112 replicate landings from 5 boats were assessed in South West Nigerian coast, off Asoroko, Lagos state (Latitude 6^oN to 6^o30’N and Longitude 3^oE to 5^oE). Fifty six (56) landings each were sampled from both wet season (May to October) and dry season (November to April) temporal scales. A fishery dependent survey method was used in bycatch quantification and identification (Kennelly et al, 1998; Kennelly, 1999). Oral interviews were conducted and samples of fishes (shrimps and bycatches) collected from fishers at landing site for identification, counting, as well as length measurement and weighing. On landing, all organisms were sorted into target shrimps, Nematopalaemon hastatus and total bycatches. All organisms in the bycatch were identified using morphometric and meristic features, they were further sorted according to families and species. The following data were collected from all the 112 landings: (i) The total weight of shrimps, (ii) The total weight of bycatch, all in kilogram, (iii) The weights, numbers and sizes of commercially important fin fishes species were taken using weighing balance and measuring board. The total weight of shrimps and bycatch species from 56 replicate landings were pooled and average taken for the wet and dry seasons. A two tailed unpaired T-test (Snedecor and Cochran, 1980) was used to test the hypothesis that the weight of total landings from the two different populations, (that is dry season and wet season landings )do not differ.

Twenty five species belonging to 20 families of fishes constitute the by-catch species incidentally caught and killed in coastal artisanal shrimp beam trawl fisheries (Table 1). The target shrimp is white shrimp, Nematopalaemon hastatus commonly called ‘crayfish’, with a carapace length range of 0.5 to 1.5cm. The important by-catch species are all juveniles in their nursery ground with a total length ranging from 4 to 30cm (Table 1). While majority of bycatch species such as Pseudotolithus elongatus, Pseudotolithus senegalensis, Ilisha africana, Trichiurus lepturus, Pentanemus quinquarius and Lutjanus dentatus occur throughout the year with the same length range (Table 1), other species such as; Arius latiscutatus, Eretmochelys imbrica and Callinectes amnicola were encountered in wet season. Species that occurred in dry season are: Sardinella aurita, Chloroscombrus chrysurus and Pseudotolithus typus. The ratios of Shrimps to bycatch species were consistently higher in favour of shrimps in all the 112 landings censused for both seasons. An average shrimp-bycatch ratio of 14:1 and 6:1 was obtained for wet and dry seasons respectively. More shrimps were caught in wet season (9054.1kg) than in dry season (3526.05kg ).

The total quantity of shrimps and by-catch combined per landings were higher in wet season (9054.1kg + 695.121kg = 9749.22kg) than dry season (3526.05kg +564.66kg = 40.90.71kg). T-test comparison (p<0.05) detected a significant difference.

Wet and dry seasonal variations in the tropical regions of the world have effect on many biological activities. This is particularly true in the feeding and spawning of fishes, which make them vulnerable to fishing gears. The weight preponderance of shrimps and by-catch landings in wet season over dry season landings and the high shrimp – bycatch ratio in favour of bycatch in dry season have importance consequences for any attempt by managers to reduce bycatch using temporal closures to trawling. It has been reported that the ideal scenario for the effectiveness of such closures is the existence of periods when bycatch is consistently large and prawn catches are consistently small (Liggins and Kennelly, 1995). In this fishery, the quantity of target shrimps caught is more in wet season (9054.1kg) than in dry season (3526.05kg). Also the shrimps – bycatch ratio of 14.1 and 6:1 for wet and dry seasons respectively indicates that dry season fishing generate more bycatch relative to the quantity of shrimps per landing. This implies that rainy season is profitable and environmentally friendly for the fishery. Fixed dry season closure may therefore be introduced to the fishery, but the problem is that it is likely to impinge too much on the bycatch livelihood trades of fishers, such that the fishery become economically unviable because this is likely to involve large scale and/or long term closure (November to April every year) and a significant loss of income to fishers. Also if dry season closure, as a management option is enforced, monitoring, control and surveillance of fishing activities for compliance are poor in a developing country like Nigeria. Still another problem is the fact that the fishery is artisanal and open access, there is no government regulation limiting the entry into the fishery. Therefore, seasonal closure to prevent imminent stock decline is not feasible in this fishery.

In recent years the concept of fisheries management have shifted from single species maximum sustainable yield (MSY) models to multiple species ecosystem approach that incorporate inter-specific selectivity. The MSY management approach therefore is not suitable for the sustainability of 25 species of fish (Table 1) incidentally caught and killed in coastal shrimp trawl fisheries studied. In respond to the regime change in fisheries management, the most applied management option throughout the majority of the world’s shrimp trawl fisheries during the past two decades focused on technological changes that involve modifications of conventional trawling gears and methods by incorporating bycatch reduction device (BRD) at the bunt to improve interspecific selectivity and so minimize bycatch of unwanted individuals (Watson et al, 1986; Averil, 1989; Kendall, 1990; Broadhurst et al, 1996). Broadhurst (2000) identified two categories of BRDs that could reduce bycatch in shrimp trawls; the first category includes those designs intended mainly to operate by exploiting behavioural differences between shrimps and fish using strategically placed funnels, horizontal and/or vertical panels as "escape windows" (Watson et al, 1986; Brewer et al, 1998) or panels of square meshes in codends (Averill, 1989; Broadhurst and Kennelly, 1994, 1996a). These designs operates on the principle that fish, unlike slower-moving benthic invertebrates have certain characteristics responses to towed trawls. The designs are simple and suitable for application to this fishery which is characterized by small wooden canoe of 7m to 9m length overall and small fishing net with mouth spread of 5.9m, height of 1.9m and codend mesh size of 10mm. The second category of modifications that could be carried out on the conventional trawl gear to reduce incidental capture and mortality of 25 species of juveniles fishes (Table 1) involves those that comprise relatively simple oblique panels or grids usually located within or immediately anterior to the codend (Kendall, 1990; Andrew et al, 1973; Isaksen et al, 1992; Broodhurst and Kennelly, 1996b). Most BRDs in this category e.g. selection grid are design mainly to partition the catch mechanically according to size and to exclude those individuals that are larger than the openings in the separating panel. In this fishery, the size of target shrimps (0.5cm to 1.5cm CL) is smaller than the size of bycatch species of commercial importance (4cm to 30cm TL, Table 1) and can therefore be selected using a rigid and/ or a flexible grid that select by size partitioning.

From the result in tables one and two, it is observed that shrimp beam trawl fisheries in coastal water generates more bycatch than any other fishery in Nigeria. Hall et al (2000) reported that coastal shrimp trawl fishery generate more bycatch than any known world fishery and presented an averaged shrimps- bycatch ratio of between 1:3 to 1:15 in the tropic. Other studies on bycatch from shrimp trawling have also given an alarming shrimp – bycatch ratios of 1:5 in temperate and sub-tropical regions and 1:10 in the tropics (Slavin, 1982; Andrew and Pepperell, 1992). These ratios may not be true in all cases of shrimp trawl fisheries. Numerically, bycatch quantity is plenty, but gravimetrically bycatch quantity is less in the shrimp fishery studied (Table 1). The reported ratios therefore may be applicable to inshore/offshore industrial shrimp trawling in which an individual bycatch species can measure between 1kg to 10kg by weight, whereas in coastal artisanal shrimp trawling, the bycatch species are all juveniles with a total length range of 4cm to 30cm (Table 1) in which 20 to 100 individual fishes constituted 1kg weight. Based on the numerical preponderance over weight of bycatch species, a shrimp – bycatch ratio of 14:1 and 6:1 for wet and dry seasons respectively obtained in this study is in contrast to the above shrimp – bycatch ratio generalization. The ratio of shrimps to bycatch in this fishery compares favourably with the report that the weight of prawns trawled in the Clarence estuary, Australia exceeds the weight of bycatch (Liggins and Kennelly, 1995).

This work was funded by the International Foundation for Science (IFS), Sweden, grant No. A/3160-1. I thank Dr. Matt Broadhurst, the International Project Adviser for the guidance and literature provided.