Spatial separation of spawning units of hake (Merluccius gayi peruanus) Claudia Wosnitza-Mendo and Michael Ballón HCS 190 Materials and Methods The maturity ogives presenting “bumps” came from samples limited in time to the main spawning season, i. e. August to October. The samples were taken from the landings of trawlers in Paita (5° S) and could not be assigned to specific fishing areas any more. In order to investigate our hypothesis that different spawning units were involved, we had to use available data where spatial distribution of mature females could be determined, that is the autumn surveys since May/June of Sub-samples determining maturity stages were taken for each degree of latitude covered during the survey. This allowed us to divide the samples into northern and southern sub-samples and calculate separate maturity ogives. These were calculated using a non-parametric adjustment for a logistic curve (Piñeiro and Saínza, 2003) P(l) = 1/1 + e – (a + b l) where P(l) is the proportion of mature fish at length l, and a and b the parameters of the logistic equation. The curves were fitted to the data applying the least squares approach of the ®Microsoft Excel Solver routine using the Newton algorithm. The state of gonadal maturation was recorded as immature or mature, mature individuals being either maturing, spawning, spent or resting. Introduction During the last decades we find a decrease in size-at-maturity of Peruvian hake (Merluccius gayi peruanus) females as a response to reductions in population size. But plotting the maturity ogives, “bumps” appeared in the sigmoid curve (Fig. 1) during the last investigated period ( ). The “bumps” as a fraction of small hake maturing at a bigger proportion as expected from the theoretical logistic equation, are only noticeable since after El Niño 1997/98. Trippel and Harvey (1991) subdivided 32 maturity distributions of white sucker (Catostomus commersoni) into four types. Their type III distribution, which describes “nonsuccessive increases in proportion mature with increase in age or length”, represents what we in this study call “bumps”. We hypothesize that different spatial spawning units are mixed in the same curve. Results Figure 2 shows the shape of the maturity curves of selected surveys for the areas where sufficient data were available. During austral autumn of 2001 we find large hake maturing in the northern areas (3º30’ – 6ºS) and smaller hake in the south (6º - 9ºS). We use the data of this survey later on to propose a solution to the “bumps” problem. During 2002 and 2003 hake biomass and density was very low and hake matured in all sub-areas at very small sizes. After 18 months of closure of the trawl fishery, the survey of 2004, with somewhat recovered stocks, presents two groups again, larger maturing in the north and small ones to the south. During the autumn survey of 2005 the “normal” pattern of small hake to the south and large hake to the north was observed. Due to the severe over-fishing problem affecting hake biomass at the beginning of the 2000s, not all surveys present samples where the two spawning units can be differentiated well. During the survey of autumn 2001, 1190 gonads were collected and recorded as immature or mature. Clearly separate curves of the proportion mature for each sub-area can be observed. In the sub-area from 5ºS - 6ºS we detect a mixture of small and large females in mature conditions. We exclude this area from the analysis and combine sub-areas from 3º30’S to 5ºS and from 7ºS to 9ºS, respectively. The result is presented in Fig. 3 as two separate ogives, one with Lm50 = 25.3 cm and one with Lm50 = 40.2cm. The combined value would result in 35.1 cm. Discussion The impact of the strong El Niño in combination with high fishing pressure, made the proportion of large hake (>= 55 cm) decrease to less than 0.1%, after having diminished below 1% during the moderate but long-lasting El Niño At the same time also the medium-sized hake (>= 35 cm) were drastically reduced in number (Fig. 4). As a response to the virtual lack of large and reduced number of medium-sized hake, a group of small female hake with Lm50 of about cm matured (Wosnitza-Mendo et al., 2004), evident in a more complex, polymodal structure of the length-at-maturity plot (Fig. 1). This distribution suggests that a separate treatment of the data is recommended, in our case depending on spatial separation. Mean length-at-maturity is a length based parameter which is often used to determine the proportion of mature fish in each length (age)-group in order to estimate the spawning stock biomass or to back-calculate stock size from spawning stock biomass derived from egg production studies. In these cases, the use of a combined Lm50 of the before- mentioned example from the survey of 2001 which gives a value of 35.1 cm (Fig. 3), would result in different recommendations for the assessment. In fact, we must consider the two groups (Lm50=25.3 cm and Lm50=40.2cm) as different spawning units. Among the factors contributing to the number of recruits produced by a fish population one of the most important is the population fecundity and it is mainly dependent on the size composition of the spawning stock, which may affect the spawning success. At different size (age) classes spawners may not contribute equally to reproduction (Bagenal, 1973). Martinsdottir et al. (2000) also demonstrated that egg production, based on abundance and size and age composition of spawning cod (Gadus morhua) may vary extensively among different areas. The geographical location of spawning and spatial abundance of small or large females may influence the survival of the offspring. Eggs and larvae may find different environmental scenarios, biological as well as physical, which might favor or hinder their development. The information of spatial and probable temporal variations between the spawning of different size groups of Peruvian hake will also be important to determine closures of spawning areas, which is one of the management measures used. References Bagenal, T.B Fish fecundity and its relation with stock and recruitment. Rapp. et process-Verb. de Réun. du Conseil Int. pour l’Explor. Mer, 164: Cardinale, M. and Modin, J Changes in size-at-maturity of Baltic cod (Gadus morhua) during a period of large variation in stock size and environmental conditions. Fisheries Research 41: Marteinsdottir, G., Gudmundsdottir, A., Thorsteinsson, V. and Stefansson, G Spatial variation in abundance, size composition and viable egg production of spawning cod (Gadus morhua L.) in Icelandic waters. ICES Journal of Marine Science, 57: Piñeiro, C. and Saínza, M Age estimation, growth and maturity of the European hake (Merluccius merluccius Linnaeus, 1758) from the Iberian Atlantic waters. ICES Journal of Marine Science, 60: Trippel, E.A. and Harvey, H.H Comparison of methods used to estimate age and length of fishes at sexual maturity using population of white sucker (Catostomus commersoni). Can. J. Aquat. Sci. 48: Wosnitza-Mendo, C., Guevara-Carrasco, R. and Ballón, M Possible causes of the drastic decline in mean length of Peruvian hake in Bol. Inst. Mar Peru 21 (1- 2): Fig. 4. Percentage of large (dotted line) and medium-sized (continuous line) hake from 1971 to 2005 Fig. 1. Maturity ogive of female hake (between 1998 and 2002) showing "bump" Fig. 2 Percentage of mature Peruvian female hake (Merluccius gayis peruanus) from autumn surveys between per degree of latitude. Acknowledgement This study has been carried out as part of the INCOFISH (Integrating Multiple Demands on Coastal Zones with Emphasis on Fisheries and Aquatic Ecosystems) project of the European Commission (contract number ), identifying “sustainable indicators” (WP 7). Fig. 3. Separate maturity ogives for female hake depending on sub-areas