Age and growth of Hawaiian seaturtles (Chelonia mydas): an analysis based on skeletochronology

Skeletochronological data on growth changes in humerus diameter were used to estimate the age of Hawaiian green seaturtles ranging from 28.7 to 96.0 cm straight carapace length. Two age estimation methods, correction factor and spline integration, were compared, giving age estimates ranging from 4.1 to 34.6 and from 3.3 to 49.4 yr, respectively, for the sample data. Mean growth rates of Hawaiian green seaturtles are 4–5 cm/yr in early juveniles, decline to a relatively constant rate of about 2 cm/yr by age 10 yr, then decline again to less than 1 cm/yr as turtles near age 30 yr. On average, age estimates from the two techniques differed by just a few years for juvenile turtles, but by wider margins for mature turtles. The spline-integration method models the curvilinear relationship between humerus diameter and the width of periosteal growth increments within the humerus, and offers several advantages over the correction-factor approach.

Age and size composition, growth rate, reproductive biology, and habitats of the West Australian dhufish (Glaucosoma hebraicum) and their relevance to the management of this species

Samples of the commercially and recreationally important West Australian dhufish (Glaucosoma hebraicum) were obtained from the lower west coast of Australia by a variety of methods. Fish <300 mm TL were caught over flat, hard substrata and low-lying limestone reefs, whereas larger fish were caught over larger limestone and coral reef formations. Maximum total lengths, weights, and ages were 981 mm, 15.3 kg, and 39 years, respectively, for females and 1120 mm, 23.2 kg, and 41 years, respectively, for males. The von Bertalanffy growth curves for females and males were significantly different. The values for L∞, k, and t0 in the von Bertalanffy growth equations were 929 mm, 0.111/year, and –0.141 years, respectively, for females, and 1025 mm, 0.111/year, and –0.052 years, respectively, for males. Preliminary estimates of total mortality indicated that G. hebraicum is now subjected to a level of fishing pressure that must be of concern to fishery managers. Glaucosoma hebraicum, which spawns between November and April and predominantly between December and March, breeds at a wide range of depths and is a multiple spawner. The L50’s for females and males at first maturity, i.e. 301 and 320 mm, respectively, were attained by about the end of the third year of life and are well below the minimum legal length (MLL) of 500 mm. Because females and males did not reach the MLL until the end of their seventh and sixth years of life, respectively, they would have had, on average, the opportunity of spawning during four and three spawning seasons, respectively, before they reached the MLL. However, because G. hebraicum caught in water depths >40 m typically die upon release, a MLL is of limited use for conserving this species. Alternative approaches, such as restricting fishing activity in highly fished areas, reducing daily bag limits for recreational fishermen, introducing quotas or revising specific details of certain commercial hand-line licences (or doing both) are more likely to provide effective conservation measures.

Recruitment season, size, and age of young American eels (Anguilla rostrata) entering an estuary near Beaufort, North Carolina

Net catches from 1985–86 to 1994–95 at Pivers Island, North Carolina, indicated that glass-eel stage American eels (Anguilla rostrata) were recruited to the estuary from November to early May, with peak numbers in January, February, and March. There was no declining trend in recruitment over the years of sampling. Except for one year, there was no clear seasonal decrease in mean length. But shorter glass eels were older than longer glass eels, as judged by age within the glass eel growth zone of the otolith, suggesting that smaller fish took longer to arrive. The mean age of glass eels collected from the lower estuary and a freshwater site 9.5 km upriver differed by 8.4 d (36.2 vs. 44.6, respectively). Outer increments (30–35) of the otolith growth zone of glass eels from North Carolina were significantly wider than corresponding increments of otoliths from New Brunswick. Mean total ages of North Carolina, New Jersey, and New Brunswick elvers were 175.4, 201.2, and 209.3 d, corresponding to mean lengths of 55.9, 60.9, and 58.1 mm TL, respectively. The mean durations of glass-eel growth zones (44.6, 62.3, and 69.8) were in close agreement with those from previous studies, but total ages were not. This suggested that perhaps some finer (leptocephalus stage) increments were not detected by light microscopy, differences occurred in seasonal increment deposition, or absorption of the otolith material may have taken place during metamorphosis, rendering the aging of larvae inaccurate. Judging from the long recruitment period and seasonal uniformity in both mean age and length found in our study, the spawning period of American eels may be somewhat more protracted than previously considered.

Seasonal growth of King George whiting (Sillaginodes punctata) estimated from length-at-age samples of the legal-size harvest

Samples of 11,000 King George whiting (Sillaginodes punctata) from the South Australian commercial and recreational catch, supplemented by research samples, were aged from otoliths. Samples were analyzed from three coastal regions and by sex. Most sampling was undertaken at fish processing plants, from which only fish longer than the legal minimum length were obtained. A left-truncated normal distribution of lengths at monthly age was therefore employed as model likelihood. Mean length-at-monthly-age was described by a generalized von Bertalanffy formula with sinusoidal seasonality. Likelihood standard deviation was modeled to vary allometrically with mean length. A range of related formulas (with 6 to 8 parameters) for seasonal mean length at age were compared. In addition to likelihood ratio tests of relative fit, model selection criteria were a minimum occurrence of high uncertainties (>20% SE), of high correlations (>0.9, >0.95, and >0.99) and of parameter estimates at their biological limits, and we sought a model with a minimum number of parameters. A generalized von Bertalanffy formula with t0 fixed at 0 was chosen. The truncated likelihood alleviated the overestimation bias of mean length at age that would otherwise accrue from catch samples being restricted to legal sizes.

Age, growth, mortality, and distribution of pinfish (Lagodon rhomboides) in Tampa Bay and adjacent Gulf of Mexico waters

Life-history dynamics of pinfish (Lagodon rhomboides) were examined from data derived from random station surveys conducted in Tampa Bay and adjacent Gulf of Mexico waters during 1993–97. In addition, patterns in spatial distribution and abundance in Gulf of Mexico waters were investigated. Ages determined from whole otoliths ranged from 0 to 7 years, and von Bertalanffy growth models for males and females were not significantly different. Von Bertalanffy growth model parameters were L∞=219.9 mm SL, k =0.33/yr, and t0 =–1.10 years for all fish combined. High gonadosomatic indices during October–December indicated that some spawning may occur in Tampa Bay. Estimated lengths at 50% maturity were 132 mm SL for males and 131 mm SL for females. Total instantaneous mortality rates derived from the Chapman-Robson estimator ranged from 0.88 to 1.08/yr, and natural mortality was estimated to be 0.78/yr. In Gulf of Mexico waters, pinfish catch rates declined with increasing depth, and most pinfish were caught in <17 m of water. Length distributions showed that pinfish segregate by size with increasing depth.

Age and growth of the smooth dogfish (Mustelus canis) in the northwest Atlantic Ocean

The northwest Atlantic population of smooth dogfish (Mustelus canis) ranges from Cape Cod, Massachusetts, to South Carolina. Although M. canis is seasonally abundant in this region, very little is known about important aspects of its biology, such as growth and reproductive rates. In the early 1990s, commercial fishery landings of smooth dogfish dramatically increased on the east coast of the United States. This study investigated growth rates of the east coast M. canis population through analysis of growth patterns in vertebral centra. Marginal increment analysis, estimates of precision, and patterns in seasonal growth supported the use of vertebrae to age these sharks. Growth bands in vertebral samples were used to estimate ages for 894 smooth dogfish. Age-length data were used to determine von Bertalanffy growth parameters for this population: K = 0.292/yr, L∞ = 123.57 cm, and t0 = –1.94 years for females, and K = 0.440/yr, L∞ = 105.17 cm, and t0 = –1.52 years for males. Males matured at two or three years of age and females matured between four and seven years of age. The oldest age estimate for male and female samples was ten and sixteen years, respectively.

Length and age at maturity of female petrale sole (Eopsetta jordani) determined from samples collected prior to spawning aggregation

The problem of bias in female petrale sole age and length-at-maturity relationships caused by sampling from spawning aggregations was investigated. Samples were collected prior to aggregation, and histological methods were used to determine maturity status. Mature and immature fish were classified by inspecting oocytes for the presence of yolk in September, when substantial divergence in yolked and unyolked oocyte diameters had been observed. Comparison of macroscopic and microscopic assessment of maturity showed that maturity status cannot be determined accurately by using macroscopic inspection during the summer. Female petrale sole from the central Oregon coast were 50% mature at 33 cm and 5 years of age. Comparison of data from our study with data used in recent petrale sole stock assessments showed that both sampling bias and the use of samples from sea-sons when status cannot be accurately determined have likely caused errors in fitted maturity relationships.

Age and growth of the swordfish (Xiphias gladius L.) in the waters around Taiwan determined from anal-fin rays

Age and growth of the swordfish (Xiphias gladius) in Taiwan waters was studied from counts of growth bands on cross sections of the second ray of the first anal fin. Data on lower jaw fork length and weight, and samples of the anal fin of male and female swordfish were collected from three offshore and coastal tuna longline fishing ports on a monthly basis between September 1997 and March 1999. In total, 685 anal fins were collected and 627 of them (293 males and 334 females) were aged successfully. The lower jaw fork lengths of the aged individuals ranged from 83.4 to 246.6 cm for the females and from 83.3 to 206 cm for the males. The radii of the fin rays and growth bands on the cross sections were measured under a dissecting microscope equipped with an image analysis system. Trends in the monthly marginal increment ratio indicated that growth bands formed once a year. Thus, the age of each fish was deter-mined from the number of visible growth bands. Two methods were used to estimate and compare the standard and the generalized von Bertalanffy growth parameters for both males and females. The nonlinear least square estimates of the generalized von Bertalanffy growth parameters in method II, in which a power function was used to describe the relationship between ray radius and LJFL, were recommended as most acceptable. There were significant differences in growth parameters between males and females. The growth parameters estimated for females were the following: asymptotic length (L∞) = 300.66 cm, growth coefficient (K) = 0.040/yr, age at zero length (t0) = –0.75 yr, and the fitted fourth parameter (m) = –0.785. The growth parameters estimated for males were the following: asymptotic length (L∞) = 213.05 cm, growth coefficient (K) = 0.086/yr, age at zero length (t0) = –0.626 yr, and the fitted fourth parameter (m) = –0.768.