© 2004 by ICES/CIEM International Council for the Exploration of the Sea/Conseil International pour l'Exploration de la Mer
Diel vertical behaviour, predator–prey relationships, and occupation of space by jack mackerel (Trachurus murphyi) off Chile
a Institut de Recherche pour le Développement (IRD), Centre de Recherche Halieutique Méditerranéenne et Tropicale Av. Jean Monnet, BP 171, 34203 Sète Cedex, France
b Instituto de Fomento Pesquero (IFOP) Blanco 839, Casilla 8-V, Valparaíso, Chile
c Escuela de Ciencias del Mar, Universidad Católica de Valparaíso Av. Altamirano 1480, Casilla 1020, Valparaíso, Chile
*Correspondence to A. Bertrand: tel: +33 (0)4 99 57 32 00. e-mail: Arnaud.Bertrand{at}ird.fr.
In the southeastern Pacific, jack mackerel (Trachurus murphyi, Carangidae) is a heavily exploited pelagic species, and its presence in Chilean waters in autumn and winter is assumed to be mainly due to an inshore feeding migration. Predator–prey relationships are known to depend on the spatial and temporal scale of observation, but they can also be strongly affected by factors such as diel vertical migration. In studying the case of jack mackerel in detail, we used data from three acoustic surveys carried out in central Chile in 1997, 1998, and 1999. In terms of spatial occupation, jack mackerel behaviour is "atypical" behaviour, i.e. more aggregated during the night than during the day. The patterns we observed can be related to their nocturnal active foraging behaviour. Diel feeding behaviour is therefore a key factor in the aggregating behaviour of jack mackerel and its vulnerability to the purse-seine fishery that targets these nocturnal aggregations. This particular fish diel feeding behaviour also affected predator–prey relationships in relation to the spatial scale. Positive correlations at a "small" spatial scale (<7–25 km) were observed during the night when jack mackerel foraged, but not during the day. Finally, we show that prey biomass was lower where jack mackerel were abundant, which could indicate a jack mackerel top–down control on prey communities.
Keywords: aggregating behaviour, jack mackerel, predator–prey relationships, spatial patterns, vertical diel behaviour
Received 15 March 2003; accepted 30 April 2004.
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