Ole Thomas Albert

Cod in fjords and coastal waters of North Norway: distribution and variation in length and maturity at age

The distribution of cod along the Norwegian coast and in fjords from 62 � N north to the Russian border was examined using data from annual trawl surveys carried out between 1995 and 2001. Based on differences in growth zones of the otoliths, cod are traditionally classified into two types: Northeast Arctic cod and coastal cod. Both types were found throughout the area investigated. The catch rate of both increased northwards and from offshore to inshore. In a statistical model of length at age, abiotic factors such as area and year of capture explained more of the variance than biotic factors such as sex, stage of maturity, and type of cod. Length at age increased in a southward direction and was higher for cod captured offshore than for those captured inshore. In a statistical model of the proportion mature at age, area, type, and year of capture explained more of the variance than sex and depth of capture. On average, coastal cod attained 50% maturity (M50) more than a year younger than Northeast Arctic cod. Although there were relatively large differences in age at maturity between neighbouring areas, age at maturity was lowest in the south and inshore, and in general, lower inshore than offshore. As genetic analysis clearly indicates that cod in the study area consist of at least two genetically separated stocks, it is likely that the differences found here in age at M50 might have a genetic component.

Greenland halibut observed by video in front of survey trawl: behaviour, escapement, and spatial pattern

Video recordings of Greenland halibut (Reinhardtius hippoglossoides) were made at eight trawl stations in Svalbard waters in August 2002. The recordings were made down to 600 m depth using artificial light. A method for calculating actual fish length from the video image was established and the recordings were analysed with respect to length-dependent behaviour, escapement and spatial pattern. All Greenland halibut observed were either lying on the bottom or swimming in a horizontal position close to the bottom, and there was no tendency to schooling. Individual fish reacted in an ordered way to the approaching trawl and were herded along the ends of the ground-gear. Escapement under the ground-gear was higher for smaller fish, while some larger individuals were apparently able to escape the trawl ahead of the observed region.

Maturity classes and spawning behaviour of Greenland halibut (Reinhardtius hippoglossoides)

Abstract The maturation and spawning of Greenland halibut ( Reinhardtius hippoglossoides Walbaum) is described from six trawl surveys in the spawning area along the continental slope between North-Norway and Svalbard from October 1997 to May 1998. Different maturity classes were identified with frequency analyses of logarithmic classes of the Gonadosomatic Index. The main spawning period started in November, peaked in December and ended in late January. Mature females appeared at the spawning area in the early autumn and left during the first months of the year. Males were recorded after this with residual milt, indicating that some spawning may occur several months after the main spawning season. Observations from the fishery indicated spawning concentrations near the coast half a year after the main spawning period. Sex differences in spawning behaviour were discussed and related to the problem of representative sampling.

Improving the precision of ageing assessments for long rough dab by using digitised pictures and otolith measurements

Abstract Otoliths of long rough dab ( Hippoglossoides platessoides ) were analysed using both stereomicroscope and digitised pictures. Approximately 400 fish were aged twice by each technique. The use of digitised pictures gave consistently higher precision of age-determinations. This was shown both graphically and by use of the coefficient of variation. Age determined from digitised pictures was generally higher than that determined by using a stereomicroscope. This was particularly true for older specimens. For some individuals a change in the zoning pattern in the otoliths was observed. The occurrence of these changes was not closely correlated with the visually determined maturation index. For fish of similar size otolith radius was correlated with age. The growth rate also influenced otolith morphology. This additional information is incorporated in the outline of an objective age-determination suggested by this study.

Flatfishes of Norwegian coasts and fjords

Abstract Bottom trawl surveys in North Norwegian fjords and coastal areas, on a South Norwegian coastal bank, and along the Russian Kola coast, are used to describe distribution, species composition, individual growth, population structure, and exploitation of plaice ( Pleuronectes platessa ), long rough dab ( Hippoglossoides platessoides ), witch ( Glyptocephalus cynoglossus ), lemon sole ( Microstomus kitt ), dab ( Limanda limanda ), megrim ( Lepidorhombus whiffiagonis ), halibut ( Hippoglossus hippoglossus ) and Greenland halibut ( Reinhardtius hippoglossoides ). Based on distribution and abundance, individual species have been grouped as northern, southern or intermediate, as shallow or deep, and as associated with fjords or with coastal banks. The four most abundant flatfishes in Norwegian coastal zones, plaice, long rough dab, witch, and lemon sole, were equally or more abundant in the north compared to the south. Specimens of these species were generally larger in the north and mean length at age tended to be lower. Possible nursery areas were indicated for plaice and lemon sole.

Fecundity and recruitment variability of Northeast Arctic Greenland halibut during 1980–1998, with emphasis on 1996–1998

Abstract Because of indications of recruitment failures and a historic low spawning stock, the Northeast Arctic Greenland halibut ( Reinhardtius hippoglossoides Walbaum) stock has been strongly regulated in the 1990s. Fecundity is a link between the spawning stock and the following recruitment. This paper describes relationships between fecundity and total length for Northeast Arctic Greenland halibut for 1997 and 1998, based on samples taken in autumn on the continental slope west of the Barents Sea. A previous study describes a similar relationship for 1996. Individual fecundity is raised to a population level using stock data from XSA, and sex composition data, maturity oogives, and mean length at age from surveys covering the area of distribution. Total egg production (TEP) varied slightly around 1.0×10 11 , during 1996–1998, with the highest TEP in 1998. A back-calculating exercise estimated annual TEP during 1980–1998 in the range 0.9–3.4×10 11 , and revealed that age groups 9–12 were the main contributors to TEP.

Distribution, population structure and diet of silvery pout (Gadiculus argenteus thori J. Schmidt), poor cod (Trisopterus minutus minutus (L.)), four-bearded rockling (Rhinonemus cimbrius (L.)), and Vahl's eelpout (Lycodes vahlii gracilis Reinhardt) in the Norwegian Deep

Abstract Populations of some small, demersal roundfish in the Norwegian Deep of the northeastern North Sea were analysed. Data were collected on 3–4 bottom trawl surveys each year in four years. Both silvery pout, four-bearded rockling and Vahls eelpout were moderately abundant in parts of the Norwegian Deep. The area is deeper than the normal range of distribution of poor-cod and this species only occurred along the edges of the deepwater area. The abundance of silvery pout increased towards the northwest, whereas fourbearded rockling and Vahls eelpout were more abundant in the Skagerrak. There were some indications of seasonal or ontogenetic migrations. Only 2–3 age groups were present at a time in the silvery pout and four-bearded rockling populations, whereas seven age groups of Vahls eelpout were recorded. Pelagic or benthopelagic crustaceans formed important parts of the diets of all the four species considered. In addition, poor-cod and four-bearded rockling preyed on polychaetes, whereas Vahls...

Analysis of vertebral chemistry to assess stock structure in a deep-sea shark, Etmopterus spinax

Analysis of vertebral chemistry to assess stock structure in a deep-sea shark, Etmopterus spinax Matthew N. McMillan, Christopher Izzo, Claudia Junge, Ole Thomas Albert, Armelle Jung, and Bronwyn M. Gillanders* Southern Seas Ecology Laboratories, DX650 418, School of Biological Sciences, University of Adelaide, Adelaide 5005, Australia Institute of Marine Research, PO Box, 9294, 6404 Tromsø, Norway Des Requins et Des Hommes (DRDH), BLP/Brest-Iroise, 15 rue Dumont d’Urville, Plouzané 29860, France *Corresponding author: Tel.: þ61 8 8313 6235; Fax: þ61 (0)8 8313 4364; Email: bronwyn.gillanders@adelaide.edu.au

Automatic interpretation of otoliths using deep learning

The age structure of a fish population has important implications for recruitment processes and population fluctuations, and is key input to fisheries assessment models. The current method relies on manually reading age from otoliths, and the process is labor intensive and dependent on specialist expertise. Advances in machine learning have recently brought forth methods that have been remarkably successful in a variety of settings, with potential to automate analysis that previously required manual curation. Machine learning models have previously been successfully applied to object recognition and similar image analysis tasks. Here we investigate whether deep learning models can also be used for estimating the age of otoliths from images. We adapt a standard neural network model designed for object recognition to the task of estimating age from otolith images. The model is trained and validated on a large collection of images of Greenland halibut otoliths. We show that the model works well, and that its precision is comparable to and may even surpass that of human experts. Automating this analysis will help to improve consistency, lower cost, and increase scale of age prediction. Similar approaches can likely be used for otoliths from other species as well as for reading fish scales. The method is therefore an important step forward for improving the age structure estimates of fish populations.