Spatial and temporal pattern of jigging fleet during the fishery of squid (Illex argentinus) in San Matias gulf, Patagonia, Argentina.

ENRIQUE MARIO MORSAN; AUGUSTO CÉSAR CRESPI ABRIL

Lima, Perú

Congreso; III Simposio Intenracional en Calamares del Pacífico.; 2005

Instituto del Mar del Perú

Introduction. The biology of cephalopods poses particular problems for fishery assessment and management, both because of the poor state of knowledge of many species and of their reduced longevity (Arnold, 1979; Caddy, 1983; Voss, 1983; Bravo de Laguna, 1989; Rosenberg et al., 1990). The short life cycle offers little opportunity to adjust fishing strategy during the lifetime of any one cohort (Bravo de Laguna, 1989) and makes cephalopods particularly vulnerable to recruitment overfishing (Rosenberg et al., 1990). The most accepted harvest strategy is the opportunistic allocation of fishing effort, which requires the use of short-time scale assessment methods (Pierce & Guerra, 1995). The fishery for Illex argentinus in open waters is managed by application of methods based on modified depletion analysis (Rosemberg et al, 1990), and is usually linked to a harvest rate. In most fisheries, fish behavior and fish population dynamics are better understood than fishermen behavior or fishing fleet dynamics (Hilborn, 1985; Hilborn and Walters, 1992). In San Matías Gulf (SMG; Argentina) (41º – 42º S) a small jigger fleet targets on what is considered an extension of the summer spawning stock of Illex argentinus (Morsan et al., 1999), entering to SMG from the Continental shelf during late fall-early winter. Since 1994, 5-7 jiggers have been admitted to fish from June to September. Though, annual landings have reached a maximum of 5000 tons, there are neither stock assessment nor fleet dynamics studies. The aim of this paper is to describe the spatial dynamics of the vessels and the temporal trend of CPUE during several fishing seasons to understand the stock movements inside the gulf in order to advance to its assessment. Materials and methods. During 7 years, between 1994 and 2004, daily catches, effort and position data of the jigging fleet were obtained by onboard observers. Catch per unit effort (CPUE) was estimated for each day in kg hour-1 line-1. The observers reported the catch information so all the fleet was aware of the situation of the rest of the jiggers. For the 2003 data (the most successful and extense fishing season) geostatistical techniques were used to describe the short-time fleet movement. Results and discussion. The spatial pattern of the fleet distribution shows differences between years in relation to the temporal pattern of CPUE. When the CPUE was relatively high the whole fleet fished clumped in the areas with the highest CPUE (highest stock concentrations). On the other hand, in some years with low CPUE (i.e. 1996), the jiggers were dispersed over the gulf, searching for areas with acceptable catch (Figure 1). In the first case, the spatial structure of the fleet can be described using geostatistical techniques. In the second case, spatial autocorrelation was not detected. During year 2003 the fleet fished over a wide area compared to other years. The jiggers´ distribution was patchy in zones with higher yields until the CPUE decreased, then it was scattered, since vessels were searching to other squid concentrations until the fleet clumped again. The main consequences of this behavior are a series sequential depletion of the stock over representing the stability of the CPUE, which could lead to overfishing. Even when the spring spawning stock of Argentinean squid remain inside the SMG during the fishing season, the fleet behavior violates assumptions of the depletions methods and unable their use to estimate a harvest rate.