the humboldt current system of northern and central chile - figema

MARTIN THIEL ET AL.appeared in June, and early developmental stages were found in the plankton until December (Palma1994), supporting the suggestion that reproduction is not synchronised among females. Gallardoet al. (1994) reported the first benthic stages in March with very high densities of recently settledsquat lobsters in April. Roa et al. (1995) identified nursery areas in places with very low oxygenconcentrations from where juveniles migrate to nearby adult habitats. It is generally assumed thatfemales produce only one clutch per year, but the extended reproductive period and the apparentlyshort incubation period of 2–3 months (Palma & Arana 1997) could allow some females to producemore than one clutch per year. Fecundity has been estimated at 1800–34,000 eggs in Region VIII(Palma & Arana 1997), and most females reach sexual maturity at carapace lengths of ~25 mm.Red squat lobsters are estimated to live from 5 to 10 yr (Roa & Tapia 1998).Biology and stock estimatesAs can be seen from the preceding section, information on the biology of the nylon shrimp andthe squat lobsters is mainly based on demographic data (size at first reproduction, fecundity, sexratio, per cent ovigerous females, embryo developmental stages), but little information is availableon their behaviour. A close analysis of monthly landings suggests possible seasonal changes inbehaviour during the year and indicates that it is important to incorporate this information in stockestimates (Pérez 2005).Pérez (2005) proposed that the biomass of the nylon shrimp showed well-defined cycles ofavailability (sensu Menge 1972), which were not related to its true abundance in biomass (sensuMenge 1972). The first cycle (termed the ‘short cycle’) occurs from September to December andis characterised by a decrease in availability (minimum in October), followed by a recovery(maximum availability in December). This leads to a cycle of greater length (termed the ‘longcycle’), characterised by a decline in available biomass, reaching a minimum in April and followedby an increase again reaching a maximum in August. Reasons for these cycles are not clear, but it hasbeen suggested that they are related to the moult and reproduction. In this sense, a sudden decreasein availability could be due to recently moulted individuals hiding from predators (Pérez 2005).The total biomass of Heterocarpus reedi in Region IV in September 1997 was estimated to beabout 5000 t, but not all of these shrimp were susceptible to fishing gear, resulting in an availablebiomass (to the fishery) of 4200 t (for details see Pérez 2005). The estimated total biomass decreasedto 2000 t in August 2000 (Week 143), representing a decline of 60% from that estimated at theinitiation of the simulation (Figure 25A). However, the percentage variation in the catch per uniteffort (CPUE) was not proportional to the decrease in abundance within periods of maximumavailability (Figure 25A). At the beginning of the study period, the CPUE was 0.59 t haul −1 , andin August 2000 this decreased to 0.45 t haul −1 , representing a decrease of only 24%. In recruitment,at each fishing station in 1997–1998 and 1998–1999 the difference between the control curves andavailability allowed the identification of two time windows for recruitment, one of lesser intensityin December and one of greater intensity extending through June and July. This seasonal variation inthe availability of the resource to the fishing gear has a direct influence on the population estimatesand consequently the determination of TACs. Therefore it was recommended to include this biologicalinformation in stock evaluation and to conduct the corresponding research cruises during periodsof maximum availability of the resource (Pérez 2005).Crustacean fisheries and economic considerationsFisheries respond not only to fluctuations in stock abundance but also to economic considerations.From the economic perspective, the annual TAC system has produced an increase in the fishing286