Catch fluctuation patterns of firefly squid (Watasenia scintillans) in Toyama Bay

Abstract

Annual yields of firefly squid (Watasenia scintillans) in Toyama Bay, Japan Sea, widely fluctuated from 473 to 3,895 tons with the average of 1,655 tons from 1982 to 2001. Hence there has been a strong desire for reliable forecasts of annual yield among fisherman. To find out keys for yield forecasts, we examined yearly, 5–day period and daily fluctuations of catch by areas, relation between daily catch and sea temperature, and body size changes of squid during 1982 to 2001. When defining a fishing season as the period from the first date to the last date with over one–ton daily catch, the maximum duration of fishing season in Toyama Bay was from early March to middle July. The potential and optimum sea temperature for squid fishing seemed 9 to 15°C and 11 to 13°C, respectively. The western part, Shinminato, and the eastern part, Namerikawa and Uozu, of Toyama Bay showed distinctively different patterns in annual and daily catches. These differences in catch pattern were considered partly due to bottom topography. When looking at the average of years with rich yields, there were 7 days lags from Shinminato to Uozu in daily catch correlation, suggesting the eastward movement of squid. Body size of squid generally increased through a fishing season. However, the detailed examination of changes in mean body size in comparison with a known growth rate of squid indicated an occurrence of several recruitments during a season every year. This was also supported by an occurrence of several peaks in daily catches for each year and area. Understandings on recruitment mechanisms would be essential for a development of reliable yield forecast.