A procedure to evaluate the effect of lag-time in studying length structure and growth rate of young fish: the case of Phycis blennoides Brunnich, 1768 (Osteichthyes: Gadiformes) in the Central Mediterranean
Keywords:trawl surveys, lag-time, growth rates, Phycis blennoides, Mediterranean
The joint analysis of data from different programs represents a good opportunity to improve knowledge about the condition of any exploited populations. This note investigates the influence of lag-time in sample characteristics (growth rate and recruitment) by following a simple and quick exploratory procedure. In order to illustrate this approach, the Greater Fork-beard (Phycis blennoides Brunnich, 1768), a species with discrete recruitment pattern and available to the capture process in the first years of life, was considered . The length-frequency distributions (LFD), obtained during five spring (MEDITS - International) and successive autumn (GRUND - Italian) bottom trawl surveys, conducted from 1994 to 1998, in the Strait of Sicily and Southern Tyrrhenian Sea (Mediterranean) were analysed. The procedure was divided into three steps. Firstly, the LFD were analysed in order to estimate the mean length of the first component in each survey. Secondly, for each survey, the difference between the standardized mean lengths of the first components in the LFD of the two areas was compared with the corresponding lag-time in order to assess any systematic temporal effect in recruitment periodicity. Thirdly, for each area, the absolute and instantaneous growth rates between the spring and autumn surveys of the same year were computed on the base of the lag-time, to evaluate any difference in growth rate between the areas. The results show that the earlier the surveys begin in the Strait of Sicily, the smaller is the difference in mean length. Since the growth rates between the two areas are practically indistinguishable, the higher mean lengths observed in samples from the Strait of Sicily mainly reflect the lag-time. The reference mean growth rate found (1.03 and 0.93 cm per month in the Strait of Sicily and Southern Tyrrhenian Sea, respectively) agreed with the literature. In spring, the recruits of 0 group represent up to 90% of the whole trawl catch, and the smallest length class observed (4.5-5.0 cm) is close to the size of transition from the pelagic to the demersal one. The proposed exploratory procedure shows that systematic differences between the two investigated sub-areas in mean size and growth rate suggested by actual data are strongly reduced after having included the lag-time in the analysis.
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