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Article Excerpt
Introduction
Sand sole, Psettichthys melanostictus, is a common nearshore pleuronectid flatfish in the northeast Pacific Ocean. Also known as fringe sole, spotted flounder, or sand flounder, this species is often caught recreationally from shore, and it also makes up a small part of commercial trawl catches (Kramer et al., 1995). Commercial landings of sand sole in California, Oregon, and Washington have brought in over $11 million between 1981 and 2004 (PacFIN, 2005). In comparison, Dover sole, Microstomus pacificus, the most abundant commercial flatfish on the west coast of the United States, was valued at $200 million in the same period.
Although the sand sole's overall economic value is low, it consistently commands a high price per pound, with only three other flatfish species commanding a higher price (California halibut, Paralichthys californicus; Pacific halibut, Hippoglossus stenolepis; and starry flounder, Platichthys stellatus). As a comparatively minor commercial and recreational species, and with little data available, sand sole has never been the subject of a formal stock assessment, nor is one likely to be conducted in the near future.
The sand sole ranges from the Bering Sea (Allen and Smith, 1988) to Redondo Beach, southern California (Fitch and Schultz, 1978). It is common on sandy bottoms at depths <70 m (Kramer et al., 1995), but it has been taken as deep as 325 m (Allen and Smith, 1988).
Previous studies have described the growth and life history of sand sole larvae and juveniles (Hickman, 1959; Sommani, 1969), but little has been reported about the adult life history of this species. Virtually all previous sand sole studies have been conducted in Oregon, Washington, and Canada, but little is known about its California populations. Furthermore, no studies have been published on the fisheries for this species.
This study was conducted to determine the age, growth, spawning season, life history, and sex ratio of California sand sole populations and to compare them to published values from other areas. In addition, we describe the fishery to evaluate whether there was any evidence of a decline in abundance and determine where the fisheries were conducted. We also reviewed the scientific literature on sand sole and compared our results to previous studies. We also present, to the best of our knowledge, the first attempt at age validation.
Materials and Methods
Collection of Samples
Samples of adult sand sole were collected by trawl net off central California in the vicinity of Monterey Bay, during 39 tows conducted between November 2001 and March 2005. Sampling equipment consisted of a commercial bottom trawl net with a 4.6 m vertical opening, 41.2 m footrope, and a 10.2 cm mesh in the codend. A net liner with 1.3 cm mesh was inserted in the codend. The net was towed at a ground speed of about 2.5 knots, typically for 1 h. Tows were made at various depths between 20 and 600 m.
Fish were processed onshore to determine fork length (FL) to the nearest millimeter, weight in grams, sex, and maturity. Sagittal otoliths were removed and stored dry in coin envelopes. Sex was determined by dissection and examination of the gonads. Our previous experience suggested that determining maturity state for flatfish is difficult without careful microscopic examination. To assign a maturity state, we classified females as mature if eggs were clearly visible and could be easily separated, or if there was evidence of recent egg release (i.e. loose oocytes within the ovary). If the ovaries were small and no eggs were apparent, we classified the fish as immature. In cases where no oocytes were readily visible to the naked eye, but the ovaries were large, we classified the maturity as unknown. Males were classified as mature if either sperm ran freely or the testes were enlarged and well developed. Males were classified as immature if the testes were small and translucent. If the testes were intermediate, the maturity state was classified as unknown.
Age and Growth
To validate yearly annulus formation, we examined the whole sagittal otoliths of 226 fish under 25x magnification using ScionImage processing software. (1) Presumed annuli were measured along the dorso-ventral axis through the nucleus. Sand sole otoliths are asymmetrical with respect to one another: the left otolith has annuli oriented around the center of the otolith, whereas in the right otolith, the core is closer to the posterior end. This asymmetry between left and right otoliths was evident in all sizes of fish that we examined.
We chose to use only the left otolith for validation in this study for three reasons: 1) the annuli were generally more distinct and were easier to read on the left otolith, 2) the central axis of the annuli corresponded more closely with the central axis of the otolith, and 3) the annuli were slightly wider than those of the right otolith, reducing the likelihood of errors in measurement. We measured along the dorso-ventral axis since the annuli were more regularly shaped, than along the somewhat longer anterior-posterior axis.
Only the diameters of the first three presumed annuli (if present) were measured. We chose not to attempt to measure more than three annuli since the annuli became quite close together with age, making it difficult to obtain accurate measurements. Otoliths from male and female fish were measured separately to account for possible differences among the sexes. A marginal increment analysis was attempted, but it proved to be inconclusive due to the difficulty of determining edge type on many of the fish, particularly older ones.
This study utilized a combination of surface readings and the break-and-burn method (Chilton and Beamish, 1982) to determine the ages. Generally, fish larger than 340 mm FL were aged using the break-and-burn method, while surface readings were used for smaller fish. Surface ageing was performed at 15x magnification, while break-and-burn readings were done at 20-30x magnifications....
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