Lehodey et al.: Modelling the distribution of Beryx splendens 



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Depth (m) below 

 surface 





Figure 1 



Main seamounts fished for alfonsino, Beryx splendens, by bottom longline in New Caledonia. 



made by the longliner Humboldt from May to July 

 1991 over seamounts B, C, D, J, and K, 6 the depth 

 profile of the bottom was recorded on an echosounder 

 as the line was set. The position and the depth at 

 the exact time the terminal anchors and intermedi- 

 ate sinkers were thrown overboard were also re- 

 corded. The longliner Humboldt was equipped with 

 a Doppler sonar current indicator which provided 

 current velocity and direction at three selected 

 depths. Data recorded suggest that the current ve- 

 locity rapidly decreased with depth (Fig. 3A) and that 

 horizontal drift was probably minimal. 6 On 23 occa- 

 sions over the total of 73 longline sets, the depth of 

 the bottom was recorded at the time the buoy was 

 grabbed at the beginning of retrieval. This depth was 

 compared with the depth of the corresponding ter- 

 minal anchor recorded when the line was set. Depth 

 difference was less than 10 m for 74 % of the paired 

 comparisons (Fig. 3B) which indicates that either the 



6 Lehodey, P. 1991. Mission d'observations halieutiques sur le 

 palangrier Humboldt. Campagne de peche du 30 mai au 12 

 juillet 1991, Noumea. ORSTOM Rapp. Missions, Sci. Mer Biol., 

 Mar. 8, 44 p. 



drift of the line during sinking was limited or the 

 slope of the bottom was slight. Therefore, despite the 

 lack of a maximum depth recorder to determine the 

 actual depth of the main line (Somerton and Kik- 

 kawa, 1992), it was reasonable to assume that its 

 configuration was similar to the depth profile indi- 

 cated by the echosounder. 



The estimated depth of the sinkers was used to allo- 

 cate a mean value of depth of capture d i = 1/2 (d t +d l+l ) 

 to all the fish caught on the same 800-m line section 

 (Fig. 2). Ten meters, which is roughly half the length 

 of the branch lines, was then added to each mean 

 depth of capture d t to correct for bias introduced by 

 the fact that catches may occur at any hook level. 

 Figure 3C gives the depth variation within each sec- 

 tion. Eighty-one percent of the variation in depth is 

 less than 15 m and 929c is within the 0-25 m range. 

 This indicates that in most cases the longline was 

 nearly horizontal with the bottom. Therefore, the 

 allocation of a single depth of capture to all fish 

 caught on the same line section seems reasonable, 

 particularly as the depth of capture data were ag- 

 gregated into 25-m depth classes for analysis. Dur- 



