Title

The reproductive dynamics of swordfish Xiphias gladius L and management implications in the northwestern Atlantic

Date of Award

1997

Availability

Article

Degree Name

Doctor of Philosophy (Ph.D.)

Department

Marine Biology and Fisheries

First Committee Member

Nelson M. Ehrhardt, Committee Chair

Abstract

Swordfish is one is the most economically important highly migratory species in the northwestern Atlantic. The high level of fishing and declining catches in recent years of adult fish have prompted managers to impose regulatory measures to avoid recruitment overfishing. The lack of adequate data has prevented the development of more precise management regulations. The major objectives of this study were to determine the type of spawning pattern, the reproductive traits that would reveal the tactics for its reproductive strategy, and to recommend the use of the reproductive characteristics of the species in future management assessments.Swordfish is an indeterminate multiple spawner based on the oocyte type of development. Most of the spawning activity is centered in the area between 15$\sp\circ$N and 35$\sp\circ$N, west of 40$\sp\circ$W, as observed from the high gonad index in individuals captured in that area. No spawning activity was observed north of 35$\sp\circ$N and only occasional spawning was observed south of 15$\sp\circ$N. The major spawning season was from December to June based on the presence of females with hydrated oocytes in their ovaries. Fifty percent maturity at size (L$\sb{0.5}$) in females was estimated at 179 cm LJFL and at age was estimated at 5 years old; males attained fifty percent maturity at 129 cm LJFL. Females spawned on average every $\sim$3 days over a period of seven months. The relationship between batch fecundity $(B\sb{f}$) and length (LJFL), and at age was best expressed by an additive power model $(B\sb{f} = 1757430 + 11.57\cdot\rm LJFL\ \sp{2.88}$; $B\sb{f} = 1846990 + 77.41\cdot\rm Age\ \sp{464}).$ Mature oocytes showed no seasonal differences in oocyte dry weight and oocyte diameter between the different subareas, but showed differences in dry weight and diameter along the latitudinal gradient in the spawning area. It is proposed that differences were related to changes in temperature and salinity along the latitudinal gradient within the spawning area.The effect of fishing on the reproductive potential of the stock using the reproductive characteristics demonstrated that management advice based on SPR ratios derived from spawning stock biomass analysis are inadequate, because of the overestimation of target F. The need to have a complete understanding of swordfish movement in the North Atlantic to conduct spatially-explicit assessment is addressed on the basis of energy allocation to reproduction and growth.

Keywords

Biology, Zoology; Agriculture, Fisheries and Aquaculture

Link to Full Text

http://access.library.miami.edu/login?url=http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:9805928