Analysis of two structural hypotheses about the dynamics of shrimp ( Heterocarpus reedi ) off the coast of Chile

1. Analysis of two structural hypotheses about the dynamics of shrimp (Heterocarpus reedi)off the coast of Chile Carlos Montenegro, Mark Maunder & Maximiliano Zilleruelo

2. INTRODUCTION The chilean shrimp (Heterocarpus reedi, Decapoda, Pandalidae) inhabits the bottom in the continental shelf and upper slope off central Chile, at depths between 150 and 550 m. Fishery administration (SUBPESCA) declared the fishery as fully exploited since 1995, and established a total allowable catch of 10000 tons for 1996. Spatially, the management of the resource is based on two main zones: the northern zone, from latitude 26°03’ S. to 32° 12’ S.(including Regions II to IV), and the southern zone, from latitude 32°12’S. to 38°28’ S

3. As shrimp landings showed a notorious decrease in the period 1997-2000, during 2001 individual quotas were introduced and total catch quota was split in a temporal and spatial fractioning that included the complete closure of the southern zone of the fishery unit. Since then, alternate closures to regions in this zone have been applied annually, while total catch quota has been maintained at around 5000 ton/year. The administration of the fishery includes annual stock assessments studies, with the purposes of review the available information of the resource, know the status of the resource, analyze the implication of different management actions and recommend a total allowable catch for the next year

4. The goal of this work is to test if there are differences in the population dynamic of Heterocarpus reedi in the large zone where it inhabits in the coast of Chile. We test if is better, from the stock assessment point of view, to model the stock as one unit in the whole area, or is as two separate stocks. We use three versions of a surplus-production model, defined by the value of the shape parameter, and we test several scenarios. The scenarios differed in which parameters were shared between the two zones and consequently in the number of parameters estimated. This analysis is used to determine if the virgin biomass, the biomass at the beginning of the modeling period (1989), or the production rate at maximum production, differs between the two zones were Heterocarpus reedi inhabits. Furthermore, we will test if the data suggest a better fit to a symmetric production curve (Bmsy/B0=0.5) or an asymmetric one (Bmsy/B0≠0.5)

5. Spatial fishery structure

7. THE FISHING FLEET

9. Landings - Zone

10. CPUE

11. Gilbert’s formulation of the Pella-Tomlinson surplus production model

12. The shape parameter

13. One stock hypothesis

14. Model selection

15. Two stock

16. Model Selection

17. Results: One stock For the case of the models under the hypothesis of one stock, the best model fit was Model 1 (production function with m=2, Bmsy/B0=0.5), with different catchability coefficients for each CPUE index. For this model, we estimate virgin biomass of 63556 t, the biomass at the beginning of 1989 of 16562 t (26% of B0) and a rate of production at maximum production (r) of 0.31. For this scenario, the biomass at maximum sustainable yield (Bmsy) is 31778 t, with a maximum sustainable yield of 9857 t (Table 1).

18. Two stocks For the case of the models under the hypothesis of two stocks, the best model fit was again Model 1 (production function with m=2, Bmsy/B0=0.5), with different virgin biomass for each zone. For this model, we estimate a total virginal biomass of 63805 t (28578 t for northern zone and 35227 t for southern zone) and a total biomass at the beginning of 1989 of 16213 t (7262 and 8951 t, each zone, 25% of B0); quantities very similar to the estimations of best fit model of one stock hypothesis. Furthermore, the estimated rate of production at maximum production (r) is 0.31 for both hypotheses. For this scenario, the biomass at maximum sustainable yield (Bmsy) is 14289 t for northern zone and 17613 t for the southern zone, with maximum sustainable yields of 4500 t and 5547 each one.

19. Results: Fit to CPUE One stock Two stocks

20. Results: depletion level

21. Discussion • Processes appear to be similar between stocks • Local depletion • Main spatial process is the fishery • Share virgin biomass by habitat

22. THE END – THANK YOU VERY MUCH Carlos Montenegro División de Investigación Pesquera Instituto de Fomento Pesquero (IFOP) Blanco 839, Casilla 8v. Valparaíso. Chile. e-mail: cmontene@ifop.cl