ICES Journal of Marine Science: Journal du Conseil

Jonsson, B.; Waples, R. S.; Friedland, K. D.

doi: 10.1006/jmsc.1999.0483pmid: N/A

Among diadromous fishes, the sturgeons are particularly threatened with 13 of 15 species considered endangered or vulnerable to extinction worldwide. These species have low intrinsic rate of growth and specialized habitat requirements making them sensitive to environmental changes and human exploitation. Habitat destruction, degradation, and alternations are probably prime causes of extinction in diadromous fishes. The spread of alien species, naturally or transported by humans to new environments, may be the second most important cause. Harvest is a contributing factor to extinction risk for many diadromous populations, although there is no known example where overfishing has led directly to the extinction of species. Population viability analysis is a method which may be used for evaluating extinction risk. However, uncertainties associated with parameter estimation and model choice limit its usefulness. Avoiding short-term extinctions is difficult enough, but restoration and recovery of natural populations present a whole new array of challenges. The understanding of the genetic population structure of the species is essential in this context. Captive rearing has some use as tool to assist stock restoration. However, artificial propagation will not lead to recovery unless the fundamental problems that caused the population declines are addressed.

McDowall, R. M.

doi: 10.1006/jmsc.1999.0450pmid: N/A

Conservation issues of diadromous fishes are discussed in relation to the IUCN list of threatened species. There is still uncertainty in respect of the accurate categorization of some species and a better understanding is required of their migration strategies. Apart from the enhanced vulnerability to fisheries and environmental impacts in the river mouths and estuaries, the conservation threats appear to be largely identical to those affecting other migrating riverine species. Attention is drawn to different modes of migration in diadromous fish.

Cass, A.; Riddell, B.

doi: 10.1006/jmsc.1999.0454pmid: N/A

Population and fishery dynamics are modelled to assess performance of alternative harvest policies for chinook that spawn in western North America. The sensitivity of model performance to variations in stock productivity and patterns of environment variation are explored to evaluate the trade-off between risk of extinction and benefits from harvest. Results indicate maximum economic benefit and low risk of extinction is not likely under the present harvest policy. Even with a conservative harvest-threshold policy, wherein harvest rates are reduced at low stock size and are zero if the stock size declines below an abundance threshold, the risk of extinction is significantly reduced compared to fixed harvest rate policies. Simulated abundance forecast errors within the historical range degraded model performance only slightly. With the constraint that risk of extinction be held below an acceptable level, socio-economic indicators reveal an optimal harvest-threshold policy at a fractional harvest of surplus abundance of about 0.45 and a threshold near 100 female spawners per stock. Based on coded-wire tag results for a major hatchery population, the mean historical harvest rate (1983–1991) was 0.6.

Dempson, J. B.; Pepper, V. A.; Furey, G.; Bloom, M.; Nicholls, T.; Hoskins, G.

doi: 10.1006/jmsc.1999.0453pmid: N/A

Five-thousand wild Atlantic salmon (Salmo salar L.) smolts from Conne River, Newfoundland, were captured during their downstream migration in May 1995, and transferred to an estuarine aquaculture rearing site at Roti Bay, 23 km away. Survival was monitored throughout the experiment. The greatest mortality occurred in July, approximately 6–8 weeks following transfer. Survival of smolts to one-sea-winter salmon was 18.5%, over four times higher than the average survival of wild salmon to Conne River during the past 6 years. Growth was monitored at monthly intervals until November 1995, with additional sampling in the spring and early summer of 1996. Survivors were split into two groups and released directly into the Bay d'Espoir fjord; one group was released 27–28 June 1996, at a site approximately 7 km from the mouth of Conne River. The second group was retained at Roti Bay and released 23 July 1996. Lotek radio transmitter tags were used in evaluating the success of the experiment by tracking migration timing and subsequent distribution of cage-reared salmon throughout the Conne River system. Approximately 80% returned to Conne River and 20% strayed to other streams. Less than 50% of the surviving fish were later accounted for in local Bay d'Espoir rivers. Results are discussed in relation to the utility of this technique to enhance salmon populations.

McKinnell, S. M.; Karlström, Ö

doi: 10.1006/jmsc.1999.0456pmid: N/A

Synchronous trends in the abundance of hatchery and wild Baltic salmon populations are examined using correlation analysis. The river catch of wild adult salmon, the recruitment of age 0+ wild salmon parr, and the recapture rate of tagged hatchery smolts are positively correlated among rivers in the Baltic Sea. The spatial correlation among populations is greater than reported for other anadromous salmonids. Baltic salmon populations have undergone low frequency, high amplitude changes in abundance during the past 200 years. Recent production levels have been supported largely by hatcheries. Efforts to conserve the remaining wild populations through fisheries regulation appear to be having a positive effect on spawner abundance.

Wainwright, T. C.; Kope, R. G.

doi: 10.1006/jmsc.1999.0451pmid: N/A

In assessing extinction risk for Pacific salmon (Oncorhynchus spp.), a variety of interacting risk factors are considered, some of which can be quantified but many of which are qualitative. In addition to the common measures of population abundance and trends for individual populations, these factors include ecological and genetic effects of hatchery fish, changes in life history traits, selective effects of harvest, trends in freshwater habitat condition, and climate variation. Combining such information into an assessment of the overall degree of risk facing populations and species relies heavily on scientific judgement rather than explicit quantitative criteria. To organize multifactor information and ensure consistency, a risk-matrix method has been implemented in which individual risk factors are scored on a 5-point scale, and then combined into an overall assessment of extinction risk.

Nielsen, J. L.

doi: 10.1006/jmsc.1999.0452pmid: N/A

Changes in genetic variation across a species range may indicate patterns of population structure resulting from past ecological and demographic events that are otherwise difficult to infer and thus provide insight into evolutionary development. Genetic data is used, drawn from 11 microsatellite loci amplified from anadromous steelhead (Oncorhynchus mykiss) sampled throughout its range in the eastern Pacific Ocean, to explore population structure at the southern edge in California. Steelhead populations in this region represent less than 10% of their reported historic abundance and survive in very small populations found in fragmented habitats. Genetic data derived from three independent molecular systems (allozymes, mtDNA, and microsatellites) have shown that the southernmost populations are characterized by a relatively high genetic diversity. Two hypothetical models supporting genetic population substructure such as observed were considered: (1) range expansion with founder-flush effects and subsequent population decline; (2) a second Pleistocene radiation from the Gulf of California. Using genetic and climatic data, a second Pleistocene refugium contributing to a southern ecotone seems more feasible. These data support strong conservation measures based on genetic diversity be developed to ensure the survival of this uniquely diverse gene pool.

Reisenbichler, R. R.; Rubin, S. P.

doi: 10.1006/jmsc.1999.0455pmid: N/A

Although several studies have shown genetic differences between hatchery and wild anadromous Pacific salmon (Oncorhynchus spp.), none has provided compelling evidence that artificial propagation poses a genetic threat to conservation of naturally spawning populations. When the published studies and three studies in progress are considered collectively, however, they provide strong evidence that the fitness for natural spawning and rearing can be rapidly and substantially reduced by artificial propagation. This issue takes on great importance in the Pacific Northwest where supplementation of wild salmon populations with hatchery fish has been identified as an important tool for restoring these populations. Recognition of negative aspects may lead to restricted use of supplementation, and better conservation, better evaluation, and greater benefits when supplementation is used.

Lichatowich, J.; Mobrand, L.; Lestelle, L.

doi: 10.1006/jmsc.1999.0457pmid: N/A

Conventional wisdom holds that the depletion of Pacific salmon is a consequence of the economic development and exploitation of Pacific Northwest ecosystems, including fur trade, mining, timber harvest, grazing, irrigation, dams, municipal and industrial development, pollution, and excessive harvest. An attempt to support the fishery through artificial propagation is also recognized as a contributor to the decline. However, those proximal causes of depletion fail to adequately explain the current status of the stocks. Fishery managers have known for at least 122 years what would destroy the Pacific salmon, but having possession of that knowledge, and adding more to it, did not prevent depletion. The decline and local extinction is also a consequence of the implementation of management programmes based on assumptions that have proven to be wrong. If the century of decline is to be halted and reversed, biologists, politicians, and the public will have to undertake the difficult task of evaluating and revising those assumptions and the management programmes derived from them.

Mohn, R.

doi: 10.1006/jmsc.1999.0481pmid: N/A

The retrospective problem is a systematic inconsistency among a series of estimates of population size, or related assessment variables, based on increasing periods of data. In some stocks, this problem is of such magnitude that sequential population analyses (SPA) are deemed inapplicable. The eastern Scotian Shelf (ESS) cod fishery, which displays the retrospective problem, and simulated data are analysed to provide insight into the causes and potential solutions to this problem. The retrospective problem is shown to be a result of the traditional analysis techniques and a non-stationarity in the data used in the population analysis. A moving window analysis is developed which allows the non-stationarities to be identified and in some cases rectified. Recommendations are also made for ad hoc investigations of the data. The analysis suggests that failure to correct the retrospective problem for a stock with data like ESS cod could lead to catch-level advice that would be twice or more the intended level.