The commercial exploitation of sea trout, Salmo trutta L.

Abstract. The net and coble catch of sea trout in the River North Esk was sampled between 1977 and 1979. A comparison of the length frequency distribution of these samples with those of sea trout taken in a trap which was assumed to be unselective revealed that the net and coble fishery was very selective. Only isolated fish below 35·0 cm were encountered in the commercial catch samples, yet between 25% and 41% of the upstream migratory sea trout taken in the trap during the netting season were below this length. The sea trout in the commercial catch samples were predominantly maiden, one-sea-winter fish. The dominant age class each year was the 2·1 + group, 3·1 + fish comprising the other major age class. The freshwater age of the maiden sea trout sampled declined as the season progressed. The proportion of previously spawned fish in the samples ranged between years from 12% to 15%. The mean length and mean weight of sea trout sampled from each year's catch showed little variation, ranging from 42·9 cm to 43·3 cm and 971 g to 972 g, respectively. The ratio of males to females in the 1979 sample was 1·00:2·18. The use of morphometric characteristics, including head length, pectoral fin length and adipose fin length was investigated with the aim of distinguishing the sex of fish taken early in the season from external features alone. Although in each month the mean head length as a percentage of fork length of males was greater than that of females, this difference was not sufficient to enable the rapid determination of sex during commercial catch sampling routines. The exploitation rate of sea trout reaching the most upstream major fishing station, Morphie Dyke, was estimated from the recapture of upstream adult sea trout removed from a trap and released in the main river 2 km downstream of Morphie Dyke. The percentage exploitation rate at this fishing station was found to range between 6·4% and 12·2%, and the percentage exploitation rate for the entire net and coble fishery was at a level of approximately 30% during the period of the study. The sea trout statistics of the North Esk net and coble fishery for the period 1925–1979 were examined, and the effect of a change in fishing effort on these figures is described. Recent catch figures, although below those reported during the mid-1960s, are above the level of catches reported prior to 1952.     

Survival of sea‐water‐adapted trout,Salmo trutta L. ranched in a Danish fjord

The effect of seawater adaptation on the survival of coastally released post‐smolt trout, Salmo trutta L., was investigated by release: (1) directly (with no adaptation); (2) after retention in net pens in the sea for 29–131 days (delayed release); (3) after feeding with a high‐salt diet (12–13.5% NaCl) for 4 weeks; and (4) after a combination of (2) and (3). In total, 17 640 trout (age = 1+, 1.5 and 2+ years; mean fork lengths = 18.2–25.6 cm) were released in 14 batches in the summer or autumn months of 1986–1989. All fish were of domesticated origin and Carlin tagged. Survival and instantaneous mortality rates (total and fishing mortality) were estimated from reported recaptures. Mortality rates were estimated for: (1) the post‐smolt period; (2) the period until the legal size of capture (40 cm) was attained; and (3) for larger sea‐trout. Release with a delay of 4 weeks gave an increased survival rate. A longer adaptation period did not increase survival. On average, survival was increased by 36%. Survival was not increased by high‐salt diets. Until attainment of the legal size for capture, survival was 9.6% higher on average, with extremes as low as 1.7% and as high as 38% in individual batches.     

Blue sucker stock characteristics in the Wabash River, Indiana–Illinois, USA

Abstract  Blue sucker, Cycleptus elongatus (Lesueur), populations have declined because of habitat degradation and overharvest, and little information exists on the population characteristics and stock dynamics of this species. We examined size and age structure, mortality, growth and condition of blue sucker in the Wabash River, Indiana–Illinois, USA. A total of 356 fish collected by electric fishing from June 2003 to April 2006 ranged from 238 to 775 mm total length and 85 to 5288 g wet weight. Age ranged from 3 to 16 years and annual mortality ranged from 22 to 25%. Annual growth was greatest among younger age classes (range 48–141 mm yr⁻¹) and declined after age 6, likely at the onset of reproductive maturity. Variation in the relative condition factor (range 66–141) was heteroscedastic over the size range of fish sampled, perhaps attributable to seasonal differences in reproductive condition among mature fish.     

热带中西太平洋大眼金枪鱼的繁殖生物学特性

利用漂流金枪鱼延绳钓作业方式,于2018年6月~2019年3月对热带中西太平洋海域(WCPO,2°03''S~11°17''S,163°14''E~173°35''E)的大眼金枪鱼(Thunnus obesus)进行了取样,对其中886尾进行了繁殖生物学测定。使用广义线性模型(GLM)和有序多元Logistic回归模型,结合圆形统计方法,分析个体叉长等生物因素、渔获水深和月度等时空因素对繁殖生物学参数的影响。结果表明: 雄性和雌性平均叉长分别为132.2(±19.1) cm和125.4(±16.8) cm;取样群体中,雌雄比为0.54(±0.74),且叉长与雌雄比具有负相关关系(r=0.323, P =0.003),叉长大于140 cm后,雄性占主导;GLM模型结果显示,性腺指数(GSI)与叉长之间存在显著的二次方关系(P<0.001),即个体GSI随着叉长增加,当雄性和雌性叉长分别达到130~140 cm和140~150 cm后GSI随叉长下降;基于圆形统计发现,月度对个体GSI具有显著影响,其中雄性GSI上半年较下半年高(正弦显著,P<0.001),而雌性GSI年末和年初高于年中(余弦显著,P=0.021);有序多元Logistic模型结果显示,性腺成熟度随叉长显著增加(P<0.001),雄性性腺成熟度在5~7月较高(余弦显著,P=0.004);此外,栖息水层对雄性个体的性腺成熟度具有显著的影响(正相关,P<0.001);雄性和雌性个体50%性成熟叉长分别为96.13(?1.24) cm和104.93(?2.60) cm。研究结果为更好地了解热带WCPO大眼金枪鱼的繁殖生物学特征、科学养护热带WCPO大眼金枪鱼资源提供参考。     

Response of resident brown trout, Salmo trutta L., and rainbow trout, Oncorhynchus mykiss (Walbaum), to the stocking of hatchery-reared brown trout

Two strains of hatchery-reared adult brown trout, Salmo trutta L., [208–334 mm total length (TL); n =  591] were individually marked and released into a limestone stream. The estimated survival after one month (86%; n =  508) was comparable to that for resident brown trout and rainbow trout, Oncorhynchus mykiss (Walbaum), (89%; n =  771), but declined to 14% ( n =  83) after 8 months compared with 52% ( n =  451) for resident trout. The movement of resident trout out of stocked stretches was higher (14%) than from control sites (5%), but the population size in both individual sites and the overall study area were unaffected. The growth of resident brown trout was unaffected by stocking, but rainbow trout showed lower growth rates in stocked versus unstocked stretches both one and 8 months after stocking ( P <  0.002).     

Angling pressure, yield and catch per effort of grayling, Thymallus thymallus (L)., and brown trout, Salmo trutta L., on the rivers Glomma and Rena, southeastern Norway

Abstract Postal questionnaires, interviews and accurate reports from anglers were used to study angling activity, yield and catch per effort of grayling, Thymallus thymallus (L.), and brown trout, Salmo trutta L., on three stretches of the regulated river Glomma and two stretches of its tributary, the river Rena in southeastern Norway. Angling activity was expressed as hours angling per kilometre of river and varied from 364 (CL 245–598) to 1337 (CL 912–1991) h km^-1. Grayling and brown trout dominated the catches. The catches were estimated between 476 (CL 334–659) and 753 (CL 517–1155) grayling and brown trout per kilometre river stretch. The mean weights were estimated between 216 and 353 g for grayling, and 260 and 395 g for brown trout. The yield of grayling and brown trout was estimated between 8.7 (CL 6.1–12.4) and 46.4 (CL 34.9–62.3) kg ha^-1 yr^-1. The quotient grayling/brown trout in the catches varied between 0.41 and 5.1, and was found to be significantly negatively correlated with angling effort ( r = - 0.838, P <0.05). Catch per effort was estimated between 132 (CL 82–218) and 382 (CL 287–513) g h^-1, and was also negatively correlated to angling effort, according to: Catch per effort (g h^-1) = - 0.210 × effort (h km^-1) + 405 ( r = and 0.899, P <0.02).
Angling activity in earlier years was determined from number of permits sold where data were available. On stretches where angling activity was moderate, there was a tendency towards increase, whereas on stretches where activity had been high for several years, the activity seemed to have stagnated, indicating that a saturation had been reached.     

Dispersion of stocked brown trout, Salmo trutta L., and landlocked salmon, Salmo salar L., from stocking sites in Lake Oulujärvi, Finland

Movement and recaptures of two hatchery-reared brown trout, Salmo trutta L., stocks and landlocked salmon, Salmo salar L., released at different sites in regulated Lake Oulujärvi, were studied in relation to release site. Five groups of fish from each stock were released in approximately equal numbers. Most of the fish released in June and July were recaptured within 3 months, whereas the majority of the fish released in early winter (October and November) were caught the following spring, about 7–9 months after stocking. The release site had a significant effect on recapture rate. The results showed that fishing restrictions targeted mainly at gill net fishing are needed to preserve the stocked fish from overfishing. Significantly fewer recaptures were observed from the landlocked salmon stocking compared with brown trout. The recaptures from the landlocked salmon stocking indicated more active movement and less clumping compared with the two brown trout stocks.     

Influence of stock levels, fishing effort and environmental factors on anglers' catches of Atlantic salmon, Salmo salar L., and sea trout, Salmo trutta L.

Abstract. Monthly catches of Atlantic salmon, Salmosalar L., and sea trout, Salmo trutta L., by anglers in a west of Ireland fishery were analysed. Data were available for 50 months from the years 1971 to 1981. The most important single determinant of catch was found to be fishing effort measured in boat-days. An additional 10 boat days were found to correspond to a catch of almost 6 salmon and 23 sea trout. The relationship between catch and stock was weaker, though a relatively high catchability of sea trout at low stock levels was recorded. Slock levels, fishing effort and environmental factors accounted for much, but not all, of the variation in catch from month to month and from year to year. Most of the effect of rainfall, sunshine and water level was attributable to variations in fishing effort associated with these factors.     

Artificial reconditioning of wild sea trout, Salmo trutta L., as an enhancement option: initial results on growth and spawning success

Abstract  Following a sea trout stock collapse in 1989 and 1990, wild sea trout kelts were successfully reconditioned in brackish water using semi-moist pellets and a commercial broodstock diet. The on-growing of wild sea trout smolts was not so successful. Natural mortality was 12.9%, and 36.1% of the fish failed to adapt to the artificial diets: 82% of these were smolts. Instantaneous growth was highest from June to August (0.74%), with an overall rate for April to September of 0.51%. At the end of the first year. 36 females and 26 males matured; 34 females, with a mean fork length of 33.2 cm, were stripped yielding 22860 ova at a mean rate of 682 ova per female or 1586 ova kg^-1 body weight. The survival of green ova to eyed ova was 90–98%, and survival from green ova to hatched alevin was 95%.     

A fish selective obstacle to prevent dispersion of an unwanted fish species, based on leaping capabilities

Abstract  The European minnow, Phoxinus phoxinus (L.), constitutes a serious threat to natural brown trout, Salmo trutta L., stocks in Norway following its introduction and translocation. In the present study, the leaping capabilities of the European minnow (50–110 mm total length) and brown trout (64–255 mm total length) were investigated with the aim of constructing suitable waterfall barriers to prevent further unwanted dispersal of the minnow, but still allow passage of larger brown trout. No successful leap of minnows was recorded at 4.9–6.5 °C, even at height intervals as low as 3 cm. At 14.0–16.5 °C, minnows were able to force waterfall barriers up to 27 cm high. At 4.9–6.5 °C brown trout forced waterfall barriers up to 40 cm. Thus, building of such barriers in brooks and rivers can help prevent the dispersal of minnows, and still allow larger brown trout to pass during feeding and spawning migrations.     

Relationship between stream temperature, thermal refugia and rainbow trout Oncorhynchus mykiss abundance in arid-land streams in the northwestern United States

Abstract – Warm stream temperatures may effectively limit the distribution and abundance of Pacific salmon Oncorhynchus spp. in streams. The role of cold thermal refugia created by upwelling groundwater in mediating this effect has been hypothesized but not quantitatively described. Between June 21 and September 15, 1994, rainbow trout O. mykiss abundance within 12 northeast Oregon (USA) stream reaches was inversely correlated with mean ambient maximum stream temperatures ( r =−0.7, P <0.05). Some rainbow trout used thermal refugia (1–10 m² surface area) that were on average 3–8°C colder than ambient stream temperatures. Within the warmest reaches, high ambient stream temperatures (>22°C) persisted from mid-June through August, and on average 10–40% of rainbow trout were observed within thermal refugia during periods of midday maximum stream temperatures. Frequency of cold-water patches within reaches was not significantly associated with rainbow trout density after accounting for the influence of ambient stream temperature ( P =0.06; extra sum of squares F -test). Given prolonged high ambient stream temperatures in some reaches, the thermal refugia available in the streams we examined may be too small and too infrequent to sustain high densities of rainbow trout. However, these refugia could allow some rainbow trout to persist, although at low densities, in warm stream reaches.     

Long-term variation in piscivory in a brown trout population: effect of changes in available prey organisms

Abstract – The piscivorous behaviour in a brown trout ( Salmo trutta L.) population was studied in four discrete periods over seven decades (1917–94) in the hydroelectric reservoir Tunhovdfjord in Norway established in 1919. Piscivorous brown trout were extremely scarce prior to the introduction of two fish species Arctic charr ( Salvelinus alpinus L.) and European minnow ( Phoxinus phoxinus L.) in the 1920s. Brown trout started eating minnow at 17 cm and Arctic charr at 22 cm of length. In the 1950s, the brown trout predated extensively (60% of analysed trout) on Arctic charr and minnow. During the next four decades, the incidence of piscivorous brown trout declined to 15%, whereas the frequency of brown trout eating Arctic charr remained constant at 10%. The growth pattern, expressed as back-calculated length, demonstrated similarity in three periods (1920s, 1960s and 1990s) and improved growth in the 1950s. The improvement was addressed the impoundment of a reservoir upstream. We did not find any marked change in growth rate due to piscivority, but coefficient of variance of back-calculated lengths indicated significant variation in individual growth in age group ≥6 years from 1950 onwards. We accredit this variation to the rise of piscivorous brown trout.     

Linking habitat characteristics with juvenile density to quantify Salmo salar and Salmo trutta smolt production in the river Sävarån,Sweden

Abstract Habitat mapping along 85 km of river was related to juvenile (15 years of electric fishing) and smolt (3 years of screw‐trapping) abundance data to estimate salmon, Salmo salar L., and sea trout, Salmo trutta L., smolt production in the River Sävarån, northern Sweden. Spawning site selection by radio‐tagged salmon (n = 12) and sea trout (n = 4) was also assessed. Fifty‐one hectares of potential spawning and nursery habitat was found in the main stem river, representing 25% of the total river area. These areas were estimated to yield 1300–7580 salmon and 630–3540 sea trout smolts based on juvenile densities, equating with 3 years of screw‐trap data (2990–5080 salmon and 680–2520 trout smolts, respectively). A hypothetical maximum production of about 19 900 salmon smolts was predicted for the river at a density of 40, 0+ salmon 100 m^?2. Tracking adults during the spawning period identified optimal and potential reproductive areas.     

电子标记辅助虹鳟家系建立及快速生长家系筛选

以中国水产科学研究院黑龙江水产研究所选育的虹鳟(Oncorhynchus mykiss)优良品系G1世代为基础群体,开展了电子标记辅助的大规模虹鳟家系构建工作,建立了G2世代全同胞家系72组,并初步进行了快速生长家系的筛选工作。对不同家系鱼种进行电子芯片植入后放在同池进行生长比较,本研究中虹鳟上市日龄(约为850日龄)体质量、体长的总体家系效应都达到了极显著水平(P<0.0001);通过对不同家系间850日龄体质量和体长的家系效应显著性检验与最小二乘均值的多重比较分析,获得了682BABB、6717B7A、6828308、682A50A、6829C24、6829DC7、682A382、68284DA这8个体质量和体长性能优良的家系。上述家系将作为优先入选家系进入组建G3世代的储备亲本群体。本研究结果可以与本课题组先期开展的基于个体育种值BLUP分析的多性状复合育种工作相互印证。这将有助于提高对虹鳟生长性状进行遗传选择的准确性,同时对其他水生动物数量性状的遗传选育研究也具有借鉴意义。     

Annual variability in survival of sea-ranched Baltic salmon, Salmo salar L: significance of smolt size and marine conditions

Abstract Carlin tagging data for 1980–1991 were used to examine the influence of smolt size and feeding conditions on the post-smolt survival of ranched River Neva salmon, ( Salmo salar L.), in the the Gulf of Finland, which is the native feeding area of the stock, and in the Bothnian Sea, where the stock has been introduced. Because of better feeding conditions, the survival rates were higher and less variable in the Gulf of Finland than in the Bothnian Sea. In the Bothnian Sea, the annual variability in survival decreased and the mean value increased with increasing smolt size from the smallest (14–16 cm) to the largest (28–30 cm) size classes. The survival was positively correlated with growth rates, food resources and sea-surface temperatures. This suggests that in the Bothnian Sea the annual variability in survival is mainly because variable marine conditions affect growth rates, and, thus, the vulnerability of the post-smolts to size-dependent predation. In the Gulf of Finland, the survival advantage of large initial size and rapid growth was counteracted by size-selective post-smolt mortality from fishing. The increase in the survival rate with increasing smolt size levelled off at 22 cm, and the correlations between survival, growth and the indices of feeding conditions were mostly insignificant. For large smolts, some negative correlations were recorded, suggesting that the relative significance of mortality from fishing may even exceed that of size-dependent natural mortality. The implications of the results for management are discussed.     

Mesh selection in a sea trout, Salmo trutta L., commercial seine net fishery

Abstract The length distribution of adult sea trout, Salmo trutta L., from a commercial seine net fishery using a nominal minimum mesh size of 50 mm or 102 mm is described. The smaller mesh size captured sea trout larger than 210 mm whilst the larger mesh size caught sea trout mainly above 400 mm, with no fish below 320 mm. A sea trout length/ girth relationship is described, and coupled with biological data allowed the calculation of mesh sizes for the appropriate escapement of different sea ages of sea trout.     

Habitat utilisation,growth and predation of Cucumaria frondosa: implications for an emerging sea cucumber fishery

Abstract Biological traits of the sea cucumber Cucumaria frondosa (Gunnerus) relevant to both ecological and management perspectives were investigated in the Newfoundland region. Abundance, size and fitness of adults were maximal on hard substrates. Larvae settled ～5 weeks post‐spawning and juveniles reached a maximum length of 6 mm after 24 months. Additional size classes of sea cucumbers kept under natural environmental conditions exhibited slow seasonal growth attuned to phytoplankton blooms, indicating that ～25 years may be required to reach market size. Juveniles of the predator sea star Solaster endeca (L.) readily fed upon 1.5‐2 mm long sea cucumbers. Predation rates on adult C. frondosa by adult S. endeca were modulated by temperature and biased towards injured specimens, suggesting that trawling may exacerbate predation pressure. The combination of slow growth and high predatory pressure enhanced by fishing activities emphasises the need for precautionary management of this emerging fishery in Atlantic Canada.     

Factors affecting the fishing power of sardine fishing vessels and implications for the management of the fisheries on Lake Kariba, Zimbabwe

Abstract The factors important in determining fishing power in the Lake Kariba sardine, Limnothrissa miodon (Boulenger, 1906), fishery were identified using data from the major fishing area of the lake, the Kariba basin, for the periods 1980–1982 and 1988–1990. Comparisons were carried out using fishing power calculated from the mean catch of some standard vessels. Three methods were used for identification of the important attributes. Analysis of variance (ANOVA) was carried out for each component to compare the mean fishing power at all levels within each variable. Multiple regression analysis was used to build predictive models and to determine the factors which best predict fishing power. Factor analysis was used to ordinate vessel types and examine any vessel groupings related to fishing power. Factor analysis showed that vessel and net size were the most important variables. Vessels without engines for propulsion, of low value and without radios had less fishing power than the other vessels in the fleet. Vessels from the same company had similar fishing power, suggesting that some unmeasured variable such as quality of management linked to the fishing companies had a significant effect on catch. The factors which were important in determining catch were length of vessel, depth of the net, presence and absence of echo sounder, mobility, the type of light, number and wattage of the underwater lights in the 1988–1990 fishing period. Vessel length and net category were most important in the period 1980–1982. The models accounted for between 37.6 (1981) and 61.2% (1988) of the variation in fishing power. During the development of the fishery, there has been a change from purse seine to lift nets, and increases in the use of echo sounders, engines for the propulsion of vessels and radios, all of which could have increased effective fishing effort.     

Brown trout (Salmo trutta L.) and Arctic charr (Salvelinus alpinus (L.)) as predators on three sympatric whitefish (Coregonus lavaretus (L.)) forms in the subarctic Lake Muddusjärvi

Abstract  – Brown trout ( Salmo trutta L.) and Arctic charr ( Salvelinus alpinus (L.)) use whitefish ( Coregonus lavaretus (L.)) as their main prey in the subarctic Lake Muddusjärvi. Brown trout dwelled in littoral and pelagic habitat, whereas Arctic charr lived only in epibenthic habitat. Both species shifted to whitefish predation at a length of 20–30 cm. At this size, brown trout fed on larger whitefish than Arctic charr. Whitefish occur in three sympatric forms, differing in their habitat, ecology and morphology. Both the predators preyed primarily upon the small-sized, densely rakered whitefish form (DR), which was the most numerous whitefish form in the lake. DR used both epibenthic and pelagic habitat, whereas two sparsely rakered whitefish forms dwelled (LSR and SSR) only in epibenthic habitat: LSR in littoral and SSR in profundal areas. Sparsely rakered whitefish forms had minor importance in predator diet.     

Seasonal variation in marine growth of sea trout, Salmo trutta, in coastal Skagerrak

Abstract –  Sea trout ( Salmo trutta ) originating from small coastal streams can be found at sea throughout the year, in contrast to conspecifics from larger rivers, which typically spend the autumn and winter in fresh water. Such an extended marine stay has been observed in coastal Skagerrak. We studied the seasonal variation in marine growth of Skagerrak sea trout based on scale increment patterns and body lengths of 563 individuals captured at sea. Growth, measured as increased body length, was rapid during summer while there was no evidence for continued growth during autumn and winter. Growth decreased with increasing age of the fish. Our results suggest that coastal Skagerrak is an important feeding area for sea trout during summer, and that an extended marine stay during autumn and winter may have trade-off benefits other than somatic growth. Alternative benefits might be increased winter survival and decreased migratory costs of juvenile fish.