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Description
In recent years, a decrease in the abundance of bluefish (Pomatomus saltatrix) has been observed (Fahay et al., 1999; Munch and Conover, 2000) that has led to increased interest in a better understanding the life history of the species. Estimates of several young-of-the-year (YOY) life history characteristics, including the importance and use of estuaries as nursery habitat (Kendall and Walford, 1979) and size-dependant mortality (Hare and Cowen, 1997), are reliant upon the accuracy of growth determination. By using otoliths, it is possible to use back-calculation formulae (BCFs) to estimate the length at certain ages and stages of development for many species of fishes. Use of otoliths to estimate growth in this way can provide the same information as long-term laboratory experiments and tagging studies without the time and expense of rearing or recapturing fish. The difficulty in using otoliths in this way lies in validating that 1) there is constancy in the periodicity of the increment formation, and 2) there is no uncoupling of the relationship between somatic and otolith growth.
To date there are no validation studies demonstrating the relationship between otolith growth and somatic growth for bluefish. Daily increment formation in otoliths has been documented for larval (Hare and Cowen, 1994) and juvenile bluefish (Nyman and Conover, 1988). Hare and Cowen (1995) found age-independent variability in the ratio of otolith size to body length in early age bluefish, although these differences varied between ontogenetic stages. Furthermore, there have been no studies where an evaluation of back-calculation methods has been combined with a validation of otolith-derived lengths for juvenile bluefish.
This study uses tetracycline-marked YOY bluefish otoliths to achieve two objectives: 1) to validate the relationship between somatic and otolith growth for juvenile bluefish, and 2) to compare the effectiveness of the Dahl-Lea equation, Fraser-Lee equation, scale proportional hypothesis (SPH), and body proportional hypothesis (BPH) length back-calculation formulae.
Materials and methods
Young-of-the-year bluefish were collected by beach seine (15 m x 1.2 m x 6 mm mesh) from Clarks Cove in Buzzards Bay, Massachusetts, in July and August 2005. The fish were anesthetized with eugenol (clove oil), measured for fork length (FL), weighed, injected with tetracycline hydrochloride (75 mg/kg body weight), and individually marked with visible implant elastomer (VIE) tags (Northwest Marine Technology, Shaw Island, WA). The fish were maintained in a 970 liter flow-through seawater tank for 1 month at ambient water temperature and fed with chopped squid and fish once daily to satiation. After 32 days, the fish were remeasured, reweighed, and their sagittal otoliths were removed for analysis. In addition, 417 YOY juvenile bluefish between 62 and 182 mm FL were collected from southeastern Massachusetts during the years 2004 and 2005 and were used to develop models of the relationship between otolith radius and fork length. ANCOVA was used to compare the slopes of the otolith-size-body-size relationship between wild fish and experimental fish to determine if the experimental conditions caused uncoupling of the relationship.
Otoliths were prepared for analysis as in Oliveira (1996) with two modifications; a 0.48-mm section was... |
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