Julian Ashford

Age and growth of Scotia Sea icefish, Chaenocephalus aceratus, from the South Shetland Islands

Samples of Chaenocephalus aceratus (Lönnberg) were collected during a trawl survey carried out around the South Shetland Islands in January–February 2002. Fish were caught by commercial bottom trawl fishing down to 500 m depth, using a stratified randomized sampling design. As observed in other recent surveys within the same area, C. aceratus represented one of the predominant species. Overall, 357 specimens ranging from 13 and 67 cm (TL) were selected for the present study. Ages were estimated by counting annuli present in the sagittal otoliths, exposed by grinding and polishing along their sagittal plane. To estimate the precision of age data, we compared blind readings by readers from different institutions. The age range was 1–17 years for females and 1–15 years for males. Von Bertalanffy growth curves were fitted to the estimated age-length data for each sex. The estimated values of asymptotic length L∞ (cm) and K (year−1) were respectively 79.8 and 0.07 for females and 60.0 and 0.09 for males. The growth performance index ranged between 2 and 2.5, similar to that reported in other icefish. Sexual maturity was attained by females and males at about 10 and 9 years old respectively, at about 60% of their maximum estimated age. These results are compared with age and growth data available in the literature for C. aceratus, and discussed in the light of recent commercial exploitation.

A License Registry Improves Sampling Efficiency for a Marine Recreational Survey

Abstract Marine recreational surveys that use lists of coastal households for a sampling frame have been encouraged to improve their efficiency by using angler license registries instead. However, few studies have compared surveys of license holders and coastal households or assessed the gain in efficiency. During a survey of the recreational fishery for the blue crab Callinectes sapidus in Maryland, sport crab license holder and coastal county households were sampled by telephone. Overall, 40.5% of license holder households interviewed reported activity compared with 2.7% of coastal households interviewed. The license list showed no disadvantage in statistical properties compared with coastal households. However, the response rate was low for both lists; for the license survey, 30.5% of telephone numbers called resulted in interviews compared with 29.3% for coastal households. To improve response rates, we recommend that license registries incorporate incentives and alternative information for reaching l...

Lead–radium dating provides a framework for coordinating age estimation of Patagonian toothfish (Dissostichus eleginoides) between fishing areas

Patagonian toothfish (Dissostichus eleginoides) or ‘Chilean sea bass’ support a valuable and controversial fishery, yet their life history is not well understood and longevity estimates range from ~20 to >50 years. In this study, lead–radium dating provided valid ages for juvenile to older adult groups, which were consistent with the counting of otolith growth zones in transverse otolith sections, and longevity estimates exceeding 30 years. Lead–radium dating revealed minor biases between the radiometric age and interpretation of growth zone counting for regional fishing areas monitored by two facilities, Center for Quantitative Fisheries Ecology (CQFE) and the Central Ageing Facility (CAF), using different age estimation techniques. For CQFE, under-ageing of ~3.3 years was observed for individuals with estimated ages under 20 years. For the CAF, ages were overestimated for young fish and underestimated for the oldest fish. Lead–radium dating detected underlying problems in coordinating age estimation between geographically separated fish stocks, and provided a framework to objectively assess otolith interpretation and growth modelling between laboratories based on age-validated data.

Population Structure and Life History Connectivity of Antarctic Silverfish ( Pleuragramma antarctica ) in the Southern Ocean Ecosystem

Antarctic silverfish (Pleuragramma antarctica) are the most important pelagic forage fish on the Antarctic continental shelf. They have an exclusively pelagic life history, including cryopelagic eggs and early larvae. The discovery of extensive distributions of eggs and larvae under fast-ice inside Terra Nova Bay, and the revelation that the aggregates were stable between years, suggested dispersal over the continental shelf by older larvae and juveniles and a return as adults to spawning areas, consistent with observations from the Antarctic Peninsula of large-scale adult movement inshore. This life history hypothesis holds promise for understanding population structure in silverfish found around the Antarctic continental shelf. However, the hypothesis is challenged by inconsistencies concerning the distribution of spawning and feeding areas, a low-energy life strategy, and lack of phylogenetic diversification.

Catch Data Reported by Telephone Avoid Public Access Bias in a Marine Recreational Survey

Abstract Marine recreational surveys often intercept fishers at public access sites to address identification error and recall bias in data reported by telephone, then calculate catch as the product of the effort reported by telephone and the catch rate recorded at public access sites. However, fishers using private access are left out of the public access frame. In a survey of the recreational catch of blue crabs Callinectes sapidus in Maryland and Virginia, we compared catch calculated using catch rates from public access surveys with catch estimated from catch data reported by telephone to see whether there was evidence of public access bias. We found no identification error by blue crab fishers. However, fishers interviewed by telephone reported that 61–83% of trips were made from private access sites, and relative to catch data reported by telephone the estimates of catch calculated using catch rates were at least 25–63% larger. Catch data reported by telephone followed a delta distribution and avoid...

Private Waterfront Householders Catch Less per Trip than other Fishers: Results of a Marine Recreational Survey

Abstract Marine recreational fishery surveys that estimate catch rate by sampling fishers who use public access often assume that fishers using private access and public access catch the same amount per trip taken. Despite the risk of bias, this assumption has not been tested by comparing private-access users to the general population of fishers. During a survey of the recreational fishery for blue crabs Callinectes sapidus in Maryland, households from a list of waterfront properties were sampled by telephone at the same time as sampling of households from all coastal counties. Fishers from waterfront households overwhelmingly used private access. Waterfront households often caught a similar amount per month as coastal county households did, but catches for waterfront households were taken during more trips, resulting in catch rates that were 74% lower. Consequently, estimating catch rate by sampling only public-access users results in a bias towards overestimation of the recreational catch. We strongly r...

An annual profile of growth in the otoliths of immature Notothenia coriiceps Richardson in relation to the environment at Signy Island, Antarctica

Scanning electron microscope and image analysis techniques were used to construct a profile of otolith growth from June 1987–June 1988 in immature Notothenia coriiceps. Growth was found to fluctuate seasonally with a large decrease between August and late November 1987. Examination of stomach fullness showed that fish continued to feed during winter. Increases in absolute light levels and sea temperature during spring were found to coincide approximately with increasing growth. Conclusive evidence distinguishing between hypotheses for limitation of growth either by seawater temperature or by resource availability was not found

Physical–biological interactions influencing large toothfish over the Ross Sea shelf

Abstract We add comments to a recent series of publications in peer-reviewed journals concerning the distribution of large Antarctic toothfish (Dissostichus mawsoni) found over the inner shelf of the Ross Sea. We note that earlier fish ecologists advanced innovative hypotheses invoking physical–biological interactions with life history, and that these, far from being disproved, have been relegated by more immediately pressing management concerns. We argue that, despite the considerable advances achieved by research groups working on D. mawsoni, an understanding of distribution and abundance is incomplete without reference to the physical structure that supports their life history. We briefly consider hypotheses highlighted by the recent literature in the context of major features of the shelf circulation in the Ross Sea, in particular intrusions of modified Circumpolar Deep Water along trough systems. We suggest physical–biological interactions that may be involved and call for improvements in the monitoring programme that can help test between the competing hypotheses.

Within-Day Variability in Catch Taken by Public Access Fishers during a Recreational Fishing Survey

Abstract Access point surveys of recreational fisheries are usually stratified into weekdays and weekend days and allocate only a single sampling assignment per day sampled; this is because most variation is thought to be due to differences in angler behavior between the week and weekend and between days. However, few estimates of within-day variability are available, and this source of error has not been adequately assessed. During a 2011 survey of the recreational fishery for blue crabs Callinectes sapidus in Maryland, we used a stratified two-stage design with week and weekend strata, and multiple assignments were selected randomly within sampling days to measure within-day variability. Within-day variability accounted for 85.3–98.5% of the variation in blue crab catch between July and September, whereas 1.5–14.7% of the variation was due to between-day variability; during the months examined, there was no significant effect due to week–weekend. These results suggest the use of a simple two-stage desig...

Independent Estimates of Catch by Private and Public Access Fishers Avoid Between-Group Sources of Error in a Recreational Fishing Survey

Abstract Many recreational fishing surveys assume that fishers who use private or public property to gain access to the fishery behave similarly, but divergent behavior may result in bias and excessive error due to between-group variation. In a 2002 survey of the recreational catch of blue crab Callinectes sapidus in Maryland, we interviewed waterfront property households by telephone and sampled public access boat ramps to obtain independent estimates of private and public access catch and effort. Waterfront property households caught more crabs than fishers from public boat ramps but achieved this only by using effort over an order of magnitude larger. They maintained a consistent catch from July to September, whereas catch from public access sites showed a well-defined peak in August. Estimating catch independently for private and public access avoided bias and between-group error generated by such differences. As a result, independent estimation of catch by private and public access users is a useful ...