Uncertainty about estimating total returns of Atlantic salmon, Salmo salar to the Gander River, Newfoundland, Canada, evaluated using a fish counting fence

Abstract  For a number of rivers in Newfoundland, Atlantic salmon, Salmo salar L., is managed in relation to river-specific conservation spawning requirements. One such river is the Gander River, where between 1989 and 1999, the escapement of Atlantic salmon, a major factor in assessing the status of stock, was determined using a fish counting fence. In 2000, the counting fence was discontinued and alternative means of calculating total returns were explored. Regression and simulation methods, using relationships between total returns and salmon counts at an upstream tributary during 1989–99, formed the basis for estimates of returns for 2000, and the uncertainty around estimates. The accuracy of methods is evaluated by retrospective comparisons with actual total returns between 1989 and 1999. Estimates of total returns deviated from the actual by as much as 50–60%, depending on the method. Management implications of the approach are discussed.     

Spatial and temporal trends in abundance of Atlantic salmon, Salmo salar, in Newfoundland with emphasis on impacts of the 1992 closure of the commercial fishery

Closure of the Newfoundland commercial Atlantic salmon, Salmo salar L., fishery in 1992 was the most restrictive measure introduced to help rebuild depressed local stocks of salmon. Here, the effects of the closure are evaluated by analysing trends in abundance since 1984, and estimates of survival in both freshwater and marine environments derived from enumeration of salmon at fish counting facilities. While freshwater production of smolts generally has been maintained, marine survival rates remain low (2–10%), and highly variable. Overall, total stock size differs little from that prior to the closure of the commercial salmon fishery. Spawning escapements have increased by a factor of 2 or 3 in some rivers, but in other areas total returns are lower on average than those prior to the fishery closure. Factors other than exploitation are contributing to lack of stock recovery, resulting in continued conservation concerns.     

Influence of climate and abundance on migration timing of adult Atlantic salmon (Salmo salar) among rivers in Newfoundland and Labrador

Evidence mounts for the influence of climate variability on temporal trends in the phenology of many organisms including various species of fish. Accordingly, we examined variation in adult Atlantic salmon Salmo salar run timing in thirteen Newfoundland and Labrador rivers where returns were monitored at fishways or fish‐counting fences. Run timing varied significantly among rivers with the median date of return differing by up to 5 weeks. Duration of runs was generally short with most adults returning over a period of three to 5 weeks. A mixed model analysis incorporating a first‐order autoregressive error structure was used to generalise changes in run timing among all monitored rivers. Results indicated that the median date of return has advanced by almost 12 days over a 35‐year interval from 1978 to 2012, while several individual rivers have advanced by almost 21 days. The influence of climate on median timing was evident when the simultaneous effects of both climate and salmon abundance were controlled. We found earlier runs associated with overall warmer climate conditions on the Newfoundland and Labrador Shelf. Results contrast with those from the north‐east Atlantic where Atlantic salmon are returning later in some rivers coincident with warming climate conditions.     

Evaluation of a river classification system as a conservation measure in the management of Atlantic salmon in Insular Newfoundland

In an effort to address the decline in abundance of Atlantic salmon, Salmo salar L., in insular Newfoundland rivers, resource managers introduced a River Classification System (RCS) in 1999 to assist in the management and conservation of the resource. The RCS set daily and seasonal bag limits in the recreational fishery on a river by river basis. An analysis of the catch and effort data before and after the implementation of the RCS showed an overall decline in the number of retained salmon after the implementation of the RCS, but no change in overall effort. Upon closer examination, it was found that retention increased on Class I rivers, which had the highest allowed retention, but declined on the other class rivers. On one particular river (Harry's River), which alternated classifications from 2007 to 2010, it was found that exploitation rates increased as allowed retention increased. Newfoundland salmon anglers appear to be retention anglers and therefore the RCS was an effective tool for diverting catch and effort to specific rivers by changing retention limits.     

Snorkelling as a method for assessing spawning stock of Atlantic salmon, Salmo salar

Abstract  Reliability of underwater snorkel counts of adult Atlantic salmon, Salmo salar L., was analysed in the tributaries of the River Teno, close to the spawning period. In small (width 5–20 m) rivers, the replicated total counts of salmon were reasonably precise (CV = 5.4–8.5%), while in the medium-sized river (width 20–40 m) the precision of the counting method was considerably lower (CV = 15.3%). Low precision in a medium sized river was also observed in an experiment using marked live fish where the observation efficiency varied between 36.4% and 70.0%. In a small river, the detection efficiency of artificial fish silhouettes (test salmon) was almost perfect in pools (98%), but decreased in rapids (84%). Separate counts of males, females, grilse and large salmon were usually more variable than total counts, indicating that divers were more capable of locating a fish than properly identifying its sex and sea-age. The behaviour of adult salmon was favourable to conduct snorkel counts, as fish normally stayed still, or after hesitating, moved upstream (>95%of the cases) when encountering a diver. The high observation efficiency (>90%) and precision, favourable behaviour of salmon and congruence between snorkel counts and catch statistics in small rivers suggest that reliable data on Atlantic salmon spawning stock can be collected by snorkeling provided that the environmental conditions are suitable and the divers are experienced.     

The risk of individual fish being captured multiple times in a catch and release fishery

The proportion of angled Atlantic salmon Salmo salar L. being caught and released has increased. If individuals are repeatedly captured, this may have fish welfare consequences. Of 995 Atlantic salmon tagged during catch and release in eight Norwegian rivers, 10% were captured twice, while 3% were captured three times within the same fishing season. The probability that released salmon were captured again decreased with decreasing time left of the fishing season, decreased for larger‐sized fish and varied among rivers/years. Increased exploitation rates within the river, indicating an increased fishing pressure, strongly increased the probability that fish would be recaptured. However, the proportion of salmon caught a second time was much lower than the total exploitation rates in the same rivers (which was on average 46%). For fish tagged in the sea, the likelihood of being angled decreased with time since entering the river, which may explain why the recapture rates of caught and released fish were lower than the total exploitation rates.     

Effects of catch and release angling on Atlantic salmon, Salmo salar L., of the Conne River, Newfoundland

The effects of catch and release angling on survival of Atlantic salmon, Salmo salar L., at Conne River, Newfoundland, were investigated by retaining angled ( n =49; experimental group) and trap-caught ( n =20; control group) fish in holding cages for up to 40 days. Samples were obtained from 8 June to 4 July, 2000, and partitioned among four water temperature strata. Apart from not being angled, control fish were handled, tagged, and transferred to holding cages in a manner similar to angled salmon. Water temperatures and discharge were monitored throughout the duration of the study. Overall, 8.2% of salmon caught and released died, but 12% died among salmon angled in water temperatures ≥ 17.9 °C. No control fish died. There were no significant differences in time associated with angling, exposure to air, tagging, transfer to holding cages, nor total handling time between salmon that survived vs. those that died. Results of the study should encourage managers to continue to use catch and release as a viable tool in the management of Atlantic salmon stocks.     

Catchability of Atlantic salmon at high water temperatures: Implications for river closure temperature thresholds to catch and release angling

Warming water temperatures, combined with increased mortality following catch and release, could have synergistic consequences if rivers remain open to catch and release at high water temperatures, and catchability of fish remains similar across water temperatures. Here archived data for Atlantic salmon, Salmo salar L., were used to (a) quantify the influence of water temperature on catchability and (b) refine estimates of absolute catch and release mortality to incorporate the relationship between temperature and catchability. A significant decline in the number of Atlantic salmon caught at warmer water temperatures was found after accounting for the effects of river water height, fishing effort, run duration and year-to-year differences in fish abundance. Overall, absolute catch and release mortalities were predicted to be infrequent at cool river temperatures. At river temperatures sometimes associated with fishing closures, mortality due to the catch and release ranged from 6% to 14%. Although post-release mortality increases with water temperature, it is somewhat compensated by the reduced catchability of Atlantic salmon. Thus, the catchability component of catch and release is an integral consideration when evaluating the effectiveness of river closure temperature thresholds when managing catch and release angling.     

Possible use of taxonomic characters to identify Newfoundland and Scottish stocks of Atlantic salmon, Salmo salar L.

Abstract. Based on a pooled sample of 367 specimens of juvenile Atlanlic salmon, Salmo salar L., from eight geographically distinct home rivers, stocks from Scotland and Newfoundland can be distinguished by meristic and morphometric character sets using discriminant analysis procedures. Reduced character sets require meristic counts of pectoral fin rays, dorsal fin rays, gill rakers and vertebrae, and morphometric measurements of standard length, pectoral and pelvic fin lengths, body depth and gape width. Regional differences based on these morphometric and meristic sets are substantiated by monomorphism at each MDH locus in Scottish fish in contrast to polymorphism at the MDH-1 and MDH-3 loci in Newfoundland fish. Only the morphometric discriminant function is highly accurate (>80%) in identifying home river origins of the fish examined with the discriminating power increasing with increased fish size. These findings for juvenile fish indicate that, while meristic, morphometric and electrophoretic criteria provide a likely means for distinguishing regional fish stocks, morphometric character sets would seem to offer the best possibility for identifying home river origins of adult Atlantic salmon in mixed fisheries.     

Thermal habitat experienced by Atlantic salmon (Salmo salar L.) kelts in coastal Newfoundland waters

Thermal habitat was recorded by data storage tags (DSTs) applied to Atlantic salmon (Salmo salar L.) kelts during their seaward migration in the spring of 1998 at enumeration facilities in Highlands River, Humber River, Western Arm Brook, and Campbellton River, Newfoundland. In total, 139 DSTs were applied and data were downloaded from eleven of the recovered tags. The recovered tags had been applied at Highlands, Campbellton and Western Arm rivers and recovered in the coastal waters of Newfoundland and Québec and at the enumeration facilities at Highlands and Campbellton rivers. Water temperatures experienced by the fish were recorded for periods of 62–118 days at resolutions of 15–30 min. The data from the sea record on the DSTs were analysed for temperature patterns in relation to migration behaviour and diurnal movement of the fish. A variety of patterns were exhibited on the temperature records suggesting that the fish were behaving in various ways at different times. For Campbellton and Highlands fish over the course of some 24 h periods, night‐time temperatures changed little and were among the highest daily temperatures experienced by the fish, whereas daytime temperatures often showed dramatic and frequent shifts in temperature presumably as the fish rapidly and frequently changed depth. For the Western Arm Brook fish, rapid fluctuations in temperature occurred sometimes during the day and night without a consistent diurnal pattern. We also considered large‐scale aspects of the data by examining oceanographic conditions in relation to the temperatures recorded by the tags.