Temporal and Spatial Dynamics of a Coastal Pelagic Species, Sardinella maderensis off the Ivory Coast
Canadian Journal of Fisheries and Aquatic Sciences
P. 224 I Dakar, Senegal Mendelsc~hn, R., and P. Cury. 1989. Temporal and spatial dynamics of a coastal pelagic species, Sardinella Variations of fortnightly catch-per-unit-effort (CPUE) of Sardinella maderenris off the Ivory Coast is studied both in time and in space-time. Using multivariate time series models, lagged values of coastal sea surface temperature (ssn and salinity explain 33% of the variation of CPUE during this period. When optimal transformations are estimated, 55% of the
... 55% of the variance is explained. The optimal transformations suggest that good catch rates are a s s ciated with local upwelling or oceanic frontal structures. However, the model differs significantly from previous models that examine the CPUE for the combined catches of the fleet. The model is shown to produce reasonable forecasts for the next two years. In space-lirne, evidence is presented that suggests that the migration of the fish b e m n areas is related to niovement of structures in the ocean. The relationships between CPUE and the environment are again highly nonlinear, so that linear analpis, such as correlation analysis, will be of limited use in examining these questions. On & d i e dans le temps ainsi que dans I'espace et le temp les variations des prises par unit6 deffort (PPUE) de Sardinella maderensis obseweeS A tous les quinre jours au large de la C6te d'lvoire. Quand on utilise des d i e s de &ies chronologiques 3 plusieurs variables. les valeurs decal& de la TMS et de la salinite p r k des c&es expliqwnt 33 % de la variation des PPUE observee pendant cette p6riode. L'estimation des transformations optimales permet d'expliqwr 55 % de la variance. Ces transformations donnent 3 penser que de bons taux de capture sont associb A une remont& locale ou 3 des fronts oceaniqws. Cependant, le d i e diH& sensiblement des modeler pr&&knts qui 6tudient les PPUE en fonction des p r i m regroup& de la flottille. On montre qw le modele foumit des pr6visions raimnables pour les deux prochaines a n n k . Sur le pian spatieternpod, on m t e des faits qui portent 3 croire que la migration de ce poisson entre les secteurs est li& au d e p i a c m t de S~NCIUWS dans I'oc6an. Les relations fonctionnelles entre les PPUE et I'environnement m t une fois de plus fortement non lineaires. de sorte que I'analw lieaire, comme I'analyse de corrdation, sera dune utilite res-maderensis off the Ivory Coast. Can. I. Fish. Aquat. Sci. 46: 1686-1697. treinte pour 1'6tude de ces questions. Receiwd lune 9, 1987 Accepted May 29. 1989 (J9312) n a previous paper (Mendelssohn and Cury 1987) we studied the temporal dynamics of the combined pelagic species of I the fshes off the Ivory Coast and how environmental processes affected the combined species dynamics. ignoring my spatial dynamics. T h e is mason to believe, however. that the dynunics of the combined species. which is of the greatest hporhmx to the fleet, may differ from the dynanua of my individual s p i e s . If the biology of the individual species diffas significantly, only broad oceanographic praresses may appear to be affecting the dynamics, while different. more local 7 may be affecting a particular species. T h a t f a . in ths paper, we focus our attention on the dynamics of h e catch Rtes of the dominant pelagic species caught in this uep. hdinella maderemis. Determining the factors that affect c b g c s in pelagic fsh .bundancc. availability. and dismbution, is complicated since the fish show a preference both for attain contemponmous environmerltal conditions as well as for a particular history of the water masses and their food content (Mendclssohp a d Roy 1686 R q u le 9 juin 1987 Accept6 /e 29 mai 1989 1986; Cury and Roy 1987; Mendelssohn and Cury 1987). A purely spatial analysis would blur these feanuts. suggesting little relationship with the environment. Thus a dynamic, tcmpod approach is required. The continental shelf off the Ivory Coast is . f i e c t e d by two upwelling seasons: a main season that occm uwnd June to September and a d e r , shoner season around January and F;ebruary (Modi& 1970; Picput 1983). Both sea surface temperature and salinity in this a m vary seasonally and the intrayear variabiility in fish availability has been shown to be influenced by t h i s variability (MarcM 1967). Marchal and picaut (1977) present evidence that the strength of the local upwelling off the Ivory Coast, which is partly influenced by the configmtion of the continental shelf, induces differential pelagic fish distributions. These fish distributions will fluctuate in space due to the dynamics of the water masses (MarcM1%7; ORSTOW FRU 1976). Thus a temporal approach by iuelf also is not sufficient to explain the variability in fish availability -a spatial approach is required.