CONSERVATION OF JAPANESE WHITING THROUGH MANAGEMENT OF SWEEPING TRAMMEL NET SELECTIVITY

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CONSERVATION OF JAPANESE WHITING THROUGH MANAGEMENT OF SWEEPING TRAMMEL NET SELECTIVITY

Objectives:

Sweeping trammel net is a type of entangling net operated in the coastal area of Japan targeting Japanese whiting Sillago japonica. A doubt of the resource depletion, the simultaneous experiments on the mesh selectivity, survival, scale-loss and stress of the species after escaping through the meshes were carried out in order to establish sustainable sweeping trammel net fishery by improving its mesh selectivity successfully.

Experimental Methods:

Nine comparative fishing operations were carried out using an experimental sweeping trammel net with three different mesh sizes (27.5, 30.0 and 32.5 mm) in Tateyama Bay during the period from August to November 1996 and from April to May 1997. All the Japanese whiting caught by each different mesh size were measured for their total length and maximum body girth to the nearest millimeter. The master selectivity curve was analyzed using Kitahara’s method. The laboratory experiments for measuring survival, scale-loss and stress, were conducted from October to November 1998 using net panels of the sweeping trammel nets set on PVC pipe frames. Various sizes of the fish were put into a cylindrical tank of 135 cm dia. and 85 cm depth, and then they were simulated capture to escape through the net panels. The fish were then observed for their damage and survival during 7 days kept in the tank. To examine the stress level, blood sampling was taken from the fish that have been simulated capture. Cortisol levels of the fish samples were measured using plasma cortisol RIA kits.

Results and Discussion:

The master selectivity curve of the sweeping trammel net for Japanese whiting in terms of body girth ratio to mesh perimeter (R’) is presented in Fig. 1 and its equation is expressed as follows: s(R’) = exp{(24.60 R’³ – 111.87R’² + 164.23R’ – 74.66) – 3.87}. The curve has a peak at 1.24 relative body girth, which result that the fish with maximum body girth exceeds the mesh perimeter can pass through the inner net. The passing probability, however, decreases by increasing the body girth. All the escaped Japanese whiting survive over 7 days after simulation, while the total scale-loss was observed quite low (0.5-1%). The plasma cortisol of this fish was rather constant at the level around 40 ng/ml (Fig. 2). It concludes that improving the mesh selectivity of sweeping trammel net can be decided to be successfully without negative impact to the resource.

Fig.1. Master selectivity curve of sweeping trammel net for Japanese whiting
Fig.2. Changes in plasma cortisol levels of the escaped Japanese whiting.

KEYWORDS: Sweeping trammel net, Japanese whiting, Selectivity, Survival, Scale-loss, Stress