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Fishery Bulletin 92(4), 1994 



Horn, 1990), Humphreys et al. (1984) have shown 

 that sexual dimorphism is not responsible for the 

 existence of different size groups of alfonsino. Marked 

 declines in CPUE are observed in the Southern Hemi- 

 sphere during summer. This season corresponds to 

 the alfonsino breeding period in New Caledonian 

 waters. 11 The summer decline in catch rate could be 

 due to breeding migrations drawing the fish to 

 spawning grounds that are different from the fish- 

 ing grounds 2 (Chikuni, 1971) or to changes in vul- 

 nerability to the gear owing to seasonal physiologi- 

 cal or behavioral changes (Ricker, 1980). Data used 

 to build the models were collected on board the 

 Humboldt during the winter season. Data used to 

 validate the models were collected on board Fukuju 

 Maru and Hokko Maru at the beginning and end of 

 the warm season and during six scientific cruises, 

 five of which were carried out in summer. This sug- 

 gests that reproductive seasonality might be a fac- 

 tor in the poor temporal validation of the models. 



Other sources of temporal variation might be re- 

 lated to the environment. The ocean habitat of 

 alfonsino is not affected by continental influences but 

 is subject to hydrological fluctuations affecting the 

 deep-water masses. Some of these influences are of 

 short period such as internal waves and tidal cur- 

 rents (Eriksen, 1985; Roden, 1987), whereas others 

 recur at longer intervals such as seasonal variations 

 in ocean currents and multi-annual hydroclimatic 

 anomalies of the El Nino Southern Oscillation 

 (ENSO) (Delcroix and Henin, 1989). Such fluctua- 

 tions might have an impact on alfonsino stock struc- 

 ture, either at the recruitment stage (survival and 

 dispersal of eggs and larvae) or by modification of 

 the behavior of adults (migrations from one seamount 

 to another). However, it is difficult to demonstrate 

 the effect of these fluctuations on the presence and 

 catchability of fish. It is even more difficult to ex- 

 plain the very large differences in fishery productiv- 

 ity observed between seamounts of identical depth, 

 located only a few dozen miles apart and appearing 

 to have the same hydrological environment. Seafloor 

 topography and bottom type might account for these 

 differences, but other hypotheses can be postulated, 

 some based on the existence of a low-energy hydro- 

 thermalism (Rougerie and Wauthy, 1990) and oth- 

 ers on a hydrological anomaly called "Taylor's col- 

 umn," which could enhance species sedentarity 

 (Royer, 1978; Genin and Boehlert, 1985; Roden, 1987; 

 Dower et al., 1992; Sime-Ngando et al., 1992). Fluc- 

 tuations in intensity of this anomaly, or its disap- 

 pearance, could also be responsible for the variations 

 in productivity observed over time over a given sea- 

 mount (Boehlert and Genin, 1987). These unknown 

 environmental fluctuations cause problems in the 



interpretation of results from exploratory and com- 

 mercial fishing cruises carried out over seamounts. 

 The data collected at a given location constitute an 

 instant picture of a stock whose abundance is likely 

 to vary, irrespective of fishing effort, as a result of 

 unknown environmental variations. In other words, 

 the fertility of the seamounts could vary quite un- 

 predictably over the history of a fishery. Conse- 

 quently, modelling the distribution of a stock should 

 be confined to a relatively small temporal sampling 

 scale. 



Conclusion 



The bivariate normal model and the recursive model 

 provide complementary interpretations of length dis- 

 tribution in terms of depth of alfonsino fished on the 

 seamounts of New Caledonia by the bottom longline 

 fishery. They could be useful for the proper manage- 

 ment of fisheries over seamounts, where stocks are 

 known to be vulnerable ( Sasaki, 1986 ) because of the 

 limited habitat afforded by seamounts and the slow 

 growth rate of deep-water species. However, it would 

 appear that annual or seasonal factors, in particu- 

 lar those which account for recruitment fluctuations 

 and behavioral changes linked to reproduction, will 

 need to be incorporated into the models before they 

 can be generalized. A better understanding of the 

 functioning of the ecosystems concerned would also 

 assist in establishing the limits of generalization, 

 particularly with regard to depth and area inhab- 

 ited by alfonsino. These models could possibly be 

 adapted to other deep-water species such as certain 

 snappers and groupers. 



Acknowledgments 



We wish to thank G W. Boehlert and some of the 

 staff members of the Southwest Fisheries Science 

 Center, Honolulu Laboratory, for their helpful com- 

 ments on the draft manuscript and Tim Adams of 

 the South Pacific Commission Fisheries Programme 

 for his editorial comments. 



Literature cited 



Boehlert, G. W., and A. Genin. 



1987. A review of the effects of seamounts on bio- 

 logical processes. In B. H. Keating, P. Fryer, R. 

 Batiza, and G. W. Boehlert (eds.), Seamounts, is- 

 lands and atolls. Geophysical Monograph. 

 43:319-334. 



