New Zealand Journal of Marine and Freshwater Research abstracts
Tuna schools/aggregations in surface longline data 1993-98
David S. Kirby
Oceanic Fisheries Programme
Secretariat of the Pacific Community
BP D5
98848 Nouméa Cedex
New Caledonia
email: davidk@spc.int
Edward R. Abraham
Michael J. Uddstrom
Hudson Dean
National Institute of Water and
Atmospheric Research Limited
P.O. Box 14 901, Kilbirnie
Wellington, New Zealand
Abstract Exploratory data analysis of a high-resolution
(hook-by-hook), 6-year time series (1993-98) of observed longline catch data
for tunas was used to investigate fine-scale spatial patterns along individual
sets that may be indicative of social behaviour (i.e., schooling) and/or
the response of individual fish to favourable extrinsic conditions (i.e.,
aggregation). Methods of spatial data analysis (i.e., nearest neighbour analysis)
that have previously been applied in various other sciences (e.g., forestry
and astronomy) were used. Results indicate strong clustering of individual
tunas at characteristic scales within the set. Mean Nearest Neighbour Distances
(NNDs) were between 100 and 200 m, compared with NNDs of 200-700 m predicted
by a heterogeneous Poisson process on the same spatial domain. The results
suggest that these adult tunas were either schooling or aggregating at the
time of capture; this may therefore be related either to social behaviour
or to sub-mesoscale oceanographic features. An aggregation index was derived
from the NNDs, giving a classification method that may be used for similar
data and the development of empirical models attempting to relate patterns
in fish catch distributions to environmental variables. The success of such
models will ultimately depend on elucidating the ecological processes reflected
in oceanographic features at biologically meaningful spatial scales.
Keywords tunas; schooling; aggregation; longline; nearest
neighbour distance
M02090 Received 6 November 2002; accepted 24 April 2003; Published 5 August
2003
New Zealand Journal of Marine and Freshwater Research, 2003, Vol. 37:
633-644
0028-8330/03/3703-0633 $7.00 © The Royal Society of New Zealand
2003
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