Stock Off Research Articles

Comparison of Metazoan Parasites of Atlantic Cod, Gadus morhua, from Three Geographical Areas of Coastal Newfoundland

This study compared metazoan parasites in the digestive tract of Atlantic cod, Gadus morhua , captured from 3 geographical areas off the northeastern (NE), southeastern (SE), and southwestern (SW) coasts of Newfoundland. Samples were obtained by line trawl and frozen after capture, and the digestive tract was subsequently examined for metazoan parasites following conventional parasitological methods. Mean abundance of nematodes, but not the trematodes, and an acanthocephalan was significantly greater in cod taken from the NE than from the SE or SW areas, which were generally similar. Four species of trematodes, 3 nematodes, and an acanthocephalan, Echinorhynchus gadi , were the dominant parasites observed. Differences in mean abundance among samples from different areas might be related to the types of prey consumed rather than to environmental factors. The dominant parasites were probably acquired by cod on the NE coast after feeding on capelin, Mallotus villosus , which serve as paratenic hosts for several helminth species. Previous studies on tagging, feeding, and prevalence of 3 parasites have revealed differences in the populations of cod from the NE than from the SW or SE areas. Results from the present study, indicating geographical differences in parasite mean abundance and prevalence from the digestive tract of Atlantic cod between SW and NE areas, support the view of distinct coastal cod stocks off Newfoundland.

Status of the fishery of flying fish off northwestern coast of Sri Lanka and the effect of lunar pattern on catchability

In Kandakuliya of the Kalpitiya peninsula of Sri Lanka, a seasonal fishery for flying fish exists. The fishing operations are mainly carried out by 37 FRP boats, using gillnets of 3.4 cm and 4.5 cm stretch mesh sizes. In the present study, the fishery was investigated in two consecutive fishing seasons: from October 2002 to April 2003 and from October 2003 to April 2004. Total catch, effort and catch per unit effort (CPUE) showed seasonal variations. During the period of investigation, the highest catch was in November 2002 while the lowest catch was reported in March 2003. Fishing effort was high in November 2002 and remained at a more or less steady level till February 2003. The mean CPUE varied from 102 ± 37.8 to 224 ± 82.7 kg boat-1 day-1. Total production from this fishery during the study period, was calculated as 488.2 and 519.1 tonnes for 2002/2003 and 2003/ 2004 seasons respectively. As the CPUE has not declined during the last decade and as fishing effort has declined, it can be concluded that at present, there is no danger of overexploitation of flying fish stocks off northwestern coasts of Sri Lanka. CPUE values recorded during the full moon periods were found to be significantly lower than those recorded during the new moon period and quarter moon periods (P

Rockfish sounds and their potential use for population monitoring in the Southern California Bight

AbstractŠirović, A., Cutter, G. R., Butler, J. L., and Demer, D. A. 2009. Rockfish sounds and their potential use for population monitoring in the Southern California Bight. – ICES Journal of Marine Science, 66: 981–990. Non-lethal methods are being developed to assess and monitor the depleted rockfish stocks off southern California. For example, data from multifrequency echosounders and underwater cameras have been combined to map the dispersions and estimate the abundances of rockfish at the historical fishing sites within this region. From August to October 2007, this ship-based technique was augmented with two passive-acoustic moored recorders. One collected data at the 43 Fathom Bank for 46 days, while the other was serially deployed at 13 locations for shorter periods (1–8 d). Passive-acoustic data were analysed for the presence of rockfish sounds. Potential sources of five pulsing sounds were identified from the optically estimated species compositions at each location, as well as from known rockfish recordings collected in aquaria. All sounds had a low frequency (<900 Hz). Some were short, individual pulses (≤0.1 s), others were repetitive. A repetitive pulsing from bocaccio (Sebastes paucispinis) was the most commonly recorded sound and it occurred mainly at night. The daily calling rates at each site were quantitatively compared with the rockfish abundance estimates obtained from the active-acoustic survey, and they were positively correlated. The feasibility of using passive-acoustic tools to monitor changes in rockfish populations efficiently is discussed.

Growth, reproductive cycle, and movement of berried European lobsters (Homarus gammarus) in a local stock off southwestern Norway

Abstract Agnatt, A-L., Kristiansen T. S., and Jorstad K. E. 2007. Growth, reproductive cycle, and movement of berried kuropean lobsters (Homarus gammarus) in a local stock off southwestern Norway. – ICES Journal of Marine Science, 64: 288–297. The Norwegian fishery for the European lobster (Homarus gammarus) collapsed between 1960 and 1980, to <10% of its pre-1960 level, and since then the spawning stock seems to be too low to generate good recruitment. In 1998, a project to evaluate the feasibility and effect of protecting berried female lobsters as a management restriction was initiated. The study area selected was previously an important fishing ground in Kvitsøy off southwestern Norway, and 125 000 hatchery-reared juveniles were released between 1990 and 1994. From spring 1998 to spring 2000, a total of 942 wild and 480 cultured berried females was purchased from fishers, individually tagged with a streamer-tag, and released. The proportion of berried females in the landings varied annually from 19 to 58% for wild females, and from 22 to 44% for cultured females. By spring 2000, 23% of the tagged females had been recaptured at least once, and 3% twice or more. Average moult increment was 7 mm carapace length (CL), independent of pre-moult size in both wild and cultured females. Reproduction (spawning) and growth (moulting) alternated in a 2 y cycle for >90% of the females. A small number moulted and spawned a few weeks after hatching. More than 95% of the recaptures were taken within a radius of 1 km of the release area. Egg production varied considerably between seasons. Reproductive potential (RP) of landed berried females underestimated egg production compared with what was actually produced (AE). For the entire period, RP was estimated to be about 15.0 million eggs, and AE to be 17.2 million eggs. Cultured females contributed 27% of AE. To reduce the fishing mortality in a heavily fished and depleted population is vital. A ban on landing berried females would be a valuable first step in attempting to increase the spawning biomass.

Did over-reliance on commercial catch rate data precipitate the collapse of northern cod?

Abstract It has been suggested that a number of “lessons” can be learned from the collapse of the northern cod stock off Newfoundland and Labrador. However, not all purported lessons have been validated with available data. One lesson is thought to be that over-reliance on commercial catch rate data and an incorrect assumption regarding the functional relationship between catch rate and population size were major contributors to overestimating stock size, precipitating the collapse. The current study describes calibration approaches used in assessments, and evaluates alternative functional relationships between commercial catch rates and stock size. In addition, historical population size is re-estimated using only research vessel data and compared with estimates obtained based on both commercial catch rate and research vessel data. Calibration with commercial catch rate contributed to overestimating stock size in some years, but there is no evidence that the assumed functional relationship between commercial catch rate and population size was a significant factor in the collapse.

Greenland cod (Gadus morhua): modeling recruitment variation during the second half of the 20th century

AbstractTwo approaches were used to qualify observed variability in Greenland cod (Gadus morhua) recruitment. In the first analysis, we used the linear trend of the Greenland cod recruitment time series and climatic variables, such as air temperatures from the Denmark Strait and wind conditions off East Greenland and Southwest Greenland, to explain the interannual variation in cod recruitment off Greenland. The model resulting from this ‘trend/environmental approach’, explained 79% of the interannual variation in cod recruitment off Greenland. In the second, analytical approach, the ‘regime approach’, multiple linear regression models were used, with the input data being the time series of cod recruitment and spawning stock biomass (SSB) from Iceland and Greenland, sea surface and air temperatures around Greenland, and zonal wind components between Iceland and Greenland. Model results indicated that, during the decades between 1950 and 1990, there were three different cause–effect regimes which significantly influenced the variability of cod recruitment. The three regimes included: (a) the 1950s and 1960s, a regime with favorable sea surface temperatures and a self‐sustaining cod stock off Greenland with high SSB that produced a series of above‐average, strong year classes; (b) the 1970s and 1980s, a regime of declining SSB and recruitment, with recruitment dependent on advection from Iceland; and (c) the 1990s, when the advective potential for recruitment from the Icelandic cod stock was the only available source for replenishment of the Greenland cod stocks, because cod recruitment in Greenland waters was negligible. The three models explained 76–77% of the observed interannual variation in cod recruitment off Greenland. Both approaches suggested that advective factors were the dominant influences for cod recruitment in the ‘Iceland–Greenland System’.

Latitudinal variation in reproductive biology and growth of female Dover sole ( Microstomus pacificus) in the North Pacific, with emphasis on the Gulf of Alaska stock

Dover sole ( Microstomus pacificus) have a wide distribution throughout the North Pacific, ranging from southern Baja California (26°N) to the Gulf of Alaska (GOA, 59°N). Prior to this study, our understanding of Dover sole reproductive biology was limited to the stock off the US west coast. This study was initiated to determine the length and age at maturity for Dover sole in the GOA. We also compared the length at maturity and growth rate across latitudes to test the hypothesis that individuals near the northern extent of their range (GOA stock) mature at a larger size than individuals in the southern stock off California and Oregon. Female Dover sole (n=273) ranging in length from 198–663 mm were collected opportunistically around Kodiak Island, Alaska, (55–59°N) from February 2000 to October 2001, with sampling concentrated in the months February-April, June–July, and October–November. All ovaries were examined to determine the maturity stage using standard histological criteria. The spawning season of Dover sole in the GOA spanned at least 4 months, from February to May. Dover sole in the GOA matured at a larger size than those off California and Oregon, with an estimated length at 50% maturity (ML 50) of 439 mm. At the same fish length that nearly 100% of Dover sole from California and Oregon waters are mature, Dover sole in the GOA are just beginning to mature. The estimated age at 50% maturity (MA 50) for female Dover sole in the GOA was 6.7 years and was similar to the MA 50 for cohorts at more southern latitudes. We attribute the latitudinal variation in ML 50, combined with the similar MA 50 across latitudes, to spatial differences in growth rates.

Geographical variations in infection by larval Anisakis simplex and Contracaecum osculatum (Nematoda, Anisakidae) in walleye pollock Theragra chalcogramma stocks off Hokkaido, Japan

: Stocks of walleye pollock Theragra chalcogramma collected from: (i) the Sea of Japan (off Rebun Island and Kumaishi); (ii) the Pacific coast (off Shikabe and eastern Hokkaido); and (iii) Nemuro Strait off Hokkaido, northern Japan, were examined for anisakid nematodes during December 1999 to February 2000, and the prevalence and abundance of Anisakis simplex and Contracaecum osculatum larvae were compared among the various sampling sites for fish of the same size and age. Anisakis simplex was generally more abundant than C. osculatum. Infection by A. simplex varied between the aforementioned stocks of walleye pollock as well as within stocks, whereby fish from off Rebun Island and Nemuro Strait were infected the most, followed by those from off the Pacific coast and Kumaishi. Infection by C. osculatum differed between the host stocks, and C. osculatum was the most abundant among the fish from Nemuro Strait. The infection variations seemed to be due to differences in host growth rate, host feeding habit, and the distribution of marine mammal final hosts. The results indicate that these two larval nematodes are useful biological indicators for the population study of walleye pollock in Japanese waters.

Was over-exploitation the cause of the decline of the American plaice stock off Labrador and northeast Newfoundland?

The population of American plaice in the waters off Labrador and on the northeast Newfoundland shelf declined substantially during the mid to late 1980s, early 1990s at a time when reported catches were very low. An earlier study examined the overlap in distribution between American plaice and cod in research vessel survey data and concluded that unreported bycatch in the cod fishery could explain the decline of the Div. 2J portion of the Subarea 2+ Div. 3 K stock of American plaice. We evaluate the method proposed in that study and use reported and observer estimates of catch to investigate potential catch levels relative to survey estimates of population biomass. This method does not appear to be a good predictor of American plaice bycatch in the cod fishery as most of the regressions were not significant and it requires extrapolation well beyond the range of the data used to build the regressions. Furthermore, there was little overlap between the extent of the commercial cod fishing grounds and the distribution of American plaice in the autumn surveys. For this stock, catch to survey biomass ratios were low regardless of the source of information used to estimate catch and suggest an exploitation rate that should be well below sustainable levels. These analyses support the conclusion that fishing was not the cause of the decline in this population of American plaice.

Why the Atlantic cod (Gadus morhua) stock off eastern Nova Scotia has not recovered

An age-structured population dynamics model, incorporating interactions between Atlantic cod (Gadus morhua), the fishery, and the grey seal (Halichoerus grypus) population, was applied to the cod stock off eastern Nova Scotia (Northwest Atlantic Fisheries Organization Divisions 4Vs and 4W, commonly abbreviated to 4VsW), a stock that has dramatically declined since the late 1980s. Mortality was modeled as having three components: fishing mortality (F), seal predation (Mp), and all other sources of natural mortality (M). Specifically, M was assumed to be distinct for immature cod (ages 1–4; Mi) and mature cod (age 5 and older; Mm), and respective annual variations were estimated. Parameters estimated also included recruitment (cod abundance at age 1; R), F, and Mp. Based on our estimates of F, Mp, and M, it is unlikely that the collapse of the 4VsW cod stock can be attributed to a sudden increase in M; fishing appears to have been the primary cause for the stock's decline. However, after the moratorium on commercial fishing in 1993, increasing Mp and Mm and low R may have contributed to the failure of the 4VsW cod stock to recover.

Development of Bayesian stock assessment methods for Namibian orange roughy Hoplostethus atlanticus

Bayesian methods are useful in fisheries stock assessment because they provide a conceptually elegant and statistically rigorous approach to making decisions under uncertainty. The application of Bayesian stock assessment methods in the management of Namibian orange roughy Hoplosthethus atlanticus within the 200 mile EEZ of Namibia is reviewed. Time-series of relative abundance are short and their reliability in indicating abundance trends is uncertain. The development of informative prior probability density functions (pdfs) for the constants of proportionality ( q ) for hydro-acoustic, commercial trawl swept area, and research trawl swept area indices produced statistically consistent prior estimates of absolute abundance for each of the three grounds where more than one index of abundance was available. The posterior pdfs for stock assessment model parameters were used to account for uncertainty in evaluations of the potential consequences of alternative harvesting policies under a stock reduction model in which catch removals were assumed to account for any declines. It appears that all orange roughy stocks off Namibia have been depleted below the limit reference point (50% of long-term unfished biomass). However, the stock reduction model could not easily account for the large declines in indices on the four fishing grounds over the period from 1995 until 1999 when the informative priors for q were applied. In the 2000 stock assessment, the Bayesian procedure was updated to account formally for uncertainty in model structures that could explain the decline in abundance. The possibility of very low stock abundance could still not be discounted when these uncertainties were accounted for. Although this most recent methodology applies more statistical rigour, its complexity has hindered its acceptance in Namibia. However, if it is worth quantifying risks and uncertainties in future stock assessments for the provision of precautionary management advice, it is proposed that the assessment protocols adopted be probabilistic to account for uncertainty in model parameters, that careful attention be given to subjective judgements about their inputs and the representation of uncertainty within them, and that, where appropriate, alternative hypotheses about stock abundance and mechanisms for catchability and stock decline be taken into account. Keywords: Bayesian, orange roughy, stock assessment, structural uncertainty African Journal of Marine Science 2001, 23: 241–264

Why the Atlantic cod (<i>Gadus morhua</i>) stock off eastern Nova Scotia has not recovered

Why the Atlantic cod (<i>Gadus morhua</i>) stock off eastern Nova Scotia has not recovered

Rounding Errors to Knock Your Stocks Off

Rounding Errors to Knock Your Stocks Off

Rounding Errors to Knock Your Stocks Off

Introduction As the total value of all daily stock transactions on the Vancouver Stock Exchange kept rising in 1982 and 1983, the exchange's index kept falling. How could this be happening? The cause of the divergence between the mathematical and computed averages consisted in an erroneous algorithm to round the last digits during the computation of the index. Public documents, e.g., Quinn's article [2] include the following pieces of information. The index is the arithmetic average of the selling prices of the nearly 1400 stocks listed on the Vancouver Stock Exchange. The computation of the index started in January of 1982, with the index then pegged to 1000. In about November of 1983, exchange officials estimated that the index should have been at least at 900 and perhaps above 1000, but the computed value of the index was down near 520. The index was computed every time the price of a stock changed, which occurred about 2800 times per day. The computer carried a total of eight decimal digits during the computation, but it truncated the last two digits to display and record the index with only three decimal digits past the decimal point. Thus, if it computed the value 540.32567, then it would record 540.325 for the index. The magnitude of the discrepancy-about 520 instead of 1000 or so-indicated that the cause involved more than only an erroneous rounding of an otherwise correct computation. Indeed, with a computer carrying eight decimal digits, the relative rounding error caused by each addition cannot exceed one half of one unit in tlhe last digit, which is (1/2) X 101-8. In the worst case, if all 1399 additions of all 1400 stock prices suffered from the maximum relative error, errors would compound to at most

Estimation of the spawning stock biomass of blue grenadier (Macruronus novaezelandiae) off western Tasmania based upon the annual egg production method

Egg production of the blue grenadier, Macruronus novaezelandiae, spawning stock off western Tasmania was surveyed from June through September in 1994 and 1995. In each year, daily egg production rates were calculated for each of three surveys and a normal curve was fitted to estimate annual egg production. The CV for the egg production estimate for the survey at the peak of the 1995 season was 14%. Daily egg mortality was not significant in any survey, but a value of 5% was assumed. The spawning biomass of blue grenadier on the west coast of Tasmania was estimated to be between 83 660 and 100 073 t in 1994 and between 59 727 and 71 376 t in 1995. Confidence limits of the estimates were obtained by a bootstrap where the highly skewed egg density data were resampled. Biomass was approximately inversely proportional to incubation time. Increases or decreases in mortality resulted in slightly smaller increases or decreases of biomass respectively. Uncertainty of the sex ratio between spawning and non-spawning periods caused the largest variation in biomass estimate (~30%). The proportion of the population that spawns each year remains a further uncertainty in the estimate of total stock biomass.

A comparison between a fixed and a variable fishing mortality control rule used to manage the cod stock off southern Labrador and the east coast of Newfoundland

Scientific advice for the management of the cod stock off southern Labrador and eastern Newfoundland (northern cod) was provided based on a fixed fishing mortality rate approach from 1972 to 1988. In 1989, with declining stock size and a requirement from the fishing industry for greater quota stability, variable fishing mortality rate approaches were introduced. Departure from fixed fishing mortality approaches allowed the fishing mortality to escalate, turning what might have been a severe stock decline under a fixed fishing mortality rate into a collapse. Given the time series of recruitment estimated for the stock, the trajectory of simulated stock biomass under different fixed levels of fishing mortality is compared with what is estimated to have transpired. Two control rules which were used in providing the scientific advice for the stock, one based on fixed fishing mortality rate and one based on variable mortality rate, are explored through the use of a population model which includes random error in the estimates of abundance. The effect of systematic under-estimation of fishing mortality at the time of the assessment (the retrospective problem) is also investigated. The control rule based on a variable fishing mortality approach is shown to have a greater propensity for causing severe spawner biomass declines under the modelled conditions.

Population structure of the Australian gummy shark (Mustelus antarcticus GÜnther) inferred from allozymes, mitochondrial DNA and vertebrae counts

The gummy shark (Mustelus antarcticus) is the main target of southern Australian shark fisheries. Its stock structure was investigated through allozymes (up to 28 loci), mitochondrial DNA (up to 10 restriction enzymes) and vertebrae counts. The average heterozygosity per allozyme locus (0.099) and degree of polymorphism (0.255) was high for sharks. Composite mitochondrial DNA haplotype diversity (0.534) and mean nucleotide sequence diversity (0.16%) were also moderately high. Three of the seven polymorphic allozyme loci (CK-A*, LDH-1*, PEP*) and the mtDNA haplotypes showed significant spatial differentiation. Two genetic stocks were identified: one along the southern coast of Australia from Bunbury in Western Australia to Eden in New South Wales and one off northern New South Wales (in the region of Newcastle to Clarence River). There was some evidence for a third stock off Townsville, Queensland. The northern occurrences extend the known geographical range of this species. Vertebrae counts from Eden northwards increased, supporting the conclusion of population heterogeneity off eastern Australia.

Spatial and seasonal distribution of eggs and larvae of sandy sprat, Hyperlophus vittatus (Clupeidae), off south-western Australia

Ichthyoplankton surveys were employed to determine the distribution and spawning season of Hyperlophus vittatus off south-western Australia. Eggs and larvae of H. vittatus were sampled with 500-μm-mesh bongo-nets monthly during 1992, and less regularly during 1993, close to the beach and at 5.5 and 11 km offshore in four areas within the region of the fishery. The spatio-temporal distribution and abundance of eggs indicates that H. vittatus spawns in nearshore marine waters from May to September, with a peak in June and July. Larvae were rarer and less abundant than the eggs and therefore were less reliable indicators of spawning areas and season. Samples taken along transects across the continental shelf in July of both 1993 and 1994 indicated that H. vittatus did not spawn further than 14 km from the coast. Samples taken in July 1994 just beyond the surf zone at beaches, and at corresponding sites 5.5 km offshore, at 3.7-km intervals along 150 km of coastline indicated that H. vittatus spawns throughout the distribution of the fished stock off south-westem Australia.

Fecundity and its variability in orange roughy: effects of population density, condition, egg size, and senescence

Fecundity of orange roughy in 1987–1989 adjusted for standard length (S.L.) varied significantly between New South Wales (42 787 eggs female−1) South Australia (35 339 eggs female−1) and east Tasmania (31 085 eggs female−1). Only 10‐17% of the variability in fecundity of eastern Tasmania orange roughy was explained by s.l. in any year from 1987–1992. However, liver condition and age of the fish, in combination with s.l., explained 27% of the variation in fecundity. Fecundity declined in fish over 60 years old. It was also significantly correlated with lipid levels in the ovary, in particular, with triacylglycerol as a proportion of the total lipid fraction. Significant interannual changes in fecundity appeared to be related to the impact of fishing. From 1987–1992, the orange roughy stock off east Tasmania was reduced by 50% by the fishery, and mean fecundity increased 20% over that period. This compensatory increase in individual fecundity, combined with an apparent increase in the proportion of females spawning annually from 54 to 71%, limited the decline in the population's egg production over this period to approximately 15%.

Discrimination of American lobster (Homarus americanus) stocks off southern New England on the basis of secondary sex character allometry

Male American lobster (Homarus americanus) from inshore southern New England were discriminated from offshore males on the basis of larger relative chela size. Lobsters from Buzzards Bay (inshore) had more conspicuous sexual dimorphism than lobsters from Hydrographer Canyon (offshore), and allometric growth of male chelae was more prominent than that of female abdomens. Principal components analysis of males from combined stocks represented variability in multivariate size and relative chela size, and component score distributions of each stock were discrete. Principal components of females from both stocks comprised variability in overall size and relative abdomen size, but principal component scores overlapped extensively. Multiple-group principal component 2 was a size-free index of relative chela size that classified 96% of males to the correct stock. Multiple-group principal component 2 of females did not successfully separate stocks. Discriminant analysis of size-adjusted morphometric data classified males to stock with 100% accuracy on the basis of relative chela size. Although discrimination of size-adjusted female data classified stocks with 94% accuracy, it was less stable and not associated with onset of maturity.