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Description
Abstract--We verified the age and growth of swordfish (Xiphias gladius) by comparing ages determined from annuli in fin ray sections with daily growth increments in otoliths. Growth of swordfish of exploitable sizes is described on the basis of annuli present in cross sections of the second ray of the first anal fins of 1292 specimens (60-260 cm eye-to-fork length, EFL) caught in the region of the Hawaii-based pelagic longline fishery. The position of the initial fin ray annulus of swordfish was verified for the first time with the use of scanning electron micrographs of presumed daily growth increments present in the otoliths of juveniles. Fish growth through age 7 was validated by marginal increment analysis. Faster growth of females was confirmed, and the standard von Bertalanffy growth model was identified as the most parsimonious for describing growth in length for fish greater than 60 cm EFL. The observed growth of three fish, a year-old in size when first caught and then recaptured from 364 to 1490 days later, is consistent with modeled growth for fish of this size range. Our novel approach to verifying age and growth should increase confidence in conducting an age-structured stock assessment for swordfish in the North Pacific Ocean.
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Swordfish (Xiphias gladius) constitute an economically important fishery resource and have historically supported many large-scale commercial fisheries throughout the world's oceans. During the 1990s, however, declining catches and average sizes of swordfish in Atlantic and Mediterranean fisheries indicated possible or likely overexploitation of these populations, and the status and management of these stocks became a highly publicized issue.
Swordfish began to be a major species targeted by the Hawaii-based pelagic longline fishery in 1990 and continued as such through the late 1990s, with landings peaking at 4000-6000 t in 1991-93 (Ito et al., 1998). This longline fishery targeted swordfish within, and adjacent to, fronts of the Subtropical Convergence Zone north of Hawaii during winter and late spring (Bigelow et al., 1999). Beginning in 1999, gear restrictions were imposed and in mid-2001 a moratorium on shallow-set (swordfish style) longlining within the swordfish fishery grounds north of the equator was instated to reduce interactions of fishing gear with, and incidental take of, protected species--primarily loggerhead turtles (Caretta caretta). The moratorium was lifted in March 2004 and a regulated (by annual effort cap, gear restrictions, take limit) longline fishery was reinstated.
A preliminary stock assessment for swordfish caught in the North Pacific Ocean, based on surplus production models, was conducted in early 1999 and was updated in early 2002 to include body length composition. No age-structured assessment as yet exists for swordfish in the central North Pacific. With the subsequent re-opening of the Hawaii-based fishery, there has been renewed interest in swordfish management in the North Pacific and a recognized need for a more robust, age-structured basis for stock assessment and documentation of age distributions and growth rates. For example, Sun et al. (2005) recently assessed the population status of swordfish taken by the tuna longline fishery in the waters around Taiwan in the western North Pacific.
Our objectives in this study were the following: 1) to evaluate the accuracy and precision of our age estimates; 2) to provide several complementary data supporting a predictable periodicity (on a yearly basis) of annulus formation in cross sections of anal-fin rays of swordfish caught in the region of the Hawaii-based longline fishery and examine evidence verifying our age estimates for these specimens; and 3) to estimate sex-specific patterns of size-at-age and growth to provide input for pending age-structured assessments of swordfish stock(s) in the central North Pacific.
Materials and methods
Collections and measurements of fish
All swordfish used for age determination in this study were collected from within the general region of the Hawaii-based pelagic longline fishery (Ito et al., 1998; DeMartini et al., 2000, Fig. 2 therein). About 95% of the specimens used were caught by commercial longlines during March 1994-June 1997; specimen collections were conducted and fish measurements were recorded by National Oceanic and Atmospheric Administration (NOAA) Fisheries, Southwest Region observers. The remaining 5% of the fish were caught on research cruises conducted during April-May 1992 and 1993, September 1996, and March-April 1997. Fish were measured (eye-to-fork length, EFL, in cm) before dressing (removal of head, entrails, tail, and fins) at sea. As they were dressed, sex was scored according to macroscopic criteria and later validated by microscopic evaluation of histological preparations of gonads for subsamples of the fish (DeMartini et al., 2000). When the fins of swordfish were removed at sea, either a portion or the entire first anal fin was collected and frozen. The braincase section, including the region of the semicircular canals was collected from juvenile and young adult swordfish, either when fish were beheaded at sea or when whole frozen by catch specimens were thawed and dissected ashore. Additional larval and early young-of-year specimens (4 mm to 20 cm EFL) were collected by a neuston trawl (5-mm and 0.505-mm mesh in wings and codend, respectively) leeward of Hawaii Island during 1995-97; intact specimens were stored frozen before otolith extraction.
Laboratory processing and specimen examination
Frozen first anal fins were thawed, and the second spiny ray was selected (Berkeley and Houde, 1983), removed, and cleaned of all tissue. It was then dried in a dehydrator for 24-48 h at about 60[degrees]C, and three adjacent, transverse sections were cut with a low-speed saw. The first cut was made according to standard protocol (Ehrhardt et al., 1996) but at a newly defined position (distal end of the medial suture, hereafter "suture terminus") located about 15% of the distance beyond the basal condyle. Subsequent cuts were made distal to the first, spaced to provide wafers [approximately equal to] 1 mm thick (Uchiyama et al., 1998). The location chosen for the cuts was different from the conventional standard (i.e., at a distance above the basal condyle equal to one-half the condyle width=d/2) currently used in swordfish aging studies (Sun et al., 2002). The unconventional cut was necessary because the condyle of the second ray is often severed or lost during the removal of fins by fishermen at sea. A small series of matched (same fish) fin ray samples were cut at the suture terminus and at the d/2 positions; some of these were also cut immediately distal to the condyle (basal cut) and the numbers of annuli were counted and compared. Cross sections of rays were preserved in mounting media on glass microscope slides and stored (without cover slips) in sealed boxes. Otoliths (sagittae) were dissected from frozen larvae and young adults at the NOAA Fisheries, Pacific Islands Fisheries Science Center (Honolulu Laboratory), and stored dry after having been cleaned, rinsed with water, and dried with 95% ethanol (EtOH).
Fin ray annuli, each defined as a single pair of opaque and translucent bands completely encircling the cross-section hemisphere without partial and split checks (Ehrhardt et al., 1996; Sun et al., 2002), were enumerated. At first examination, about 1% of all cross sections were deemed unreadable and discarded. The distances separating the distal edges of the translucent band of each annulus were... |
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