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1.
A study was conducted to determine the influence of environmental change on an endoparasite, Echinorhynchus gadi (Acanthocephala) of Atlantic cod (Gadus morhua) over a 30-year period off the coast of Labrador in the north-western Atlantic, North Atlantic Fisheries Organization subareas 2J-3K. Cod, once an abundant fish species that had been commercially exploited for many decades, declined precipitously during the mid-1980s onwards. This decline was attributed to climatic changes that affected the entire food chain from zooplankton to fish, sea birds and marine mammals. A monitoring programme was introduced, sampling cod by otter trawling using research vessels. The fish, after capture, were frozen at - 20 degrees C, subsequently thawed and the digestive tract removed and examined for the parasite in 2006. Data from samples taken in 1976, 1980-81, 1986, 1990, 2000 and 2003 were compared statistically with those collected in 2006. The results indicate a decline in the prevalence and mean abundance of E. gadi in 1986 with a minimum in 2000 but increasing gradually in 2003 and 2006. These changes were coincident initially with a decline of oceanic temperature and the entire food web, including capelin (Mallotus villosus), a preferred prey of cod and primary source of E. gadi. The increase in prevalence and mean abundance of the parasite in 2006 were associated with an increase of oceanic temperature and the return of small schools of capelin to offshore areas. Cod older than 4 years harboured a greater abundance of E. gadi than younger fish, while no difference was observed between the sexes. The results suggest that the abundance of E. gadi can be useful as a bioindicator of environmental changes in the north-western Atlantic.  相似文献   

2.
The variety of life-history patterns exhibited by the five species of wrasse common in Northern Europe are reviewed. The two larger wrasse species, the ballan, Labrus bergylta Ascanius, and cuckoo, Labrus mixtus (L.), are exploited through sport angling. The three smaller species, the corkwing, Symphodus (Crenilabrus) melops (L.), rock cook, Centrolabrus exoletus (L.) and goldsinny, Ctenolabrus rupestris (L.), are being exploited by a new fishery for use as parasite cleaners of farmed salmon.
The nature of salmon farming limits the wrasse fishery to a minimum size, restricted areas and the warmer months of the year. The fishery may be expected to alter population structure through selective removal of larger fish. Removal of dominant territorial males may affect social structures and removal of nest-guarding males would reduce egg survival. Quantitative models incorporating stock size and fishery requirements are now required.  相似文献   

3.
The effects of size-selective fishing mortality on the growth of Atlantic cod ( Gadus morhuu L.) in the southern Gulf of St Lawrence were investigated and compared between: (1) a period when fishing mortality was relatively high, growth was relatively rapid, and abundance low (1967–1972 year classes): and (2) a period when fishing mortality was lower, growth was slow, and density high (1977–1982 year classes). Cod first entered the fishery at age 3 during both periods. The 1967–1972 year classes (fast growing) were fully recruited to the fishery by age 5 or 6 and the fishery removed over twice as many fish from the lower than upper quartiles of length-at-age distributions for cod 4 to 10 years old (disproportionately high exploitation of slow-growing fish). In contrast. the 1977–1982 year classes (slow growing) did not fully recruit to the fishery until age 9 or 10 and the fishery removed four times as many fish from the upper than lower quartiles of the length-at-age distributions for 4- to 10-year-old cod (disproportionately high exploitation of fast-growing fish). The reduced mean lengths-at-age of the 1977–1982 year classes compared with the 1967–I972 year-classes is consistent with the different patterns of exploitation of fast- and slow-growing fish for the two periods.  相似文献   

4.
Freshwater fishery management is treated as a dynamic system comprising environment protection and improvement, fishing, fishery resources allocation, fish stocking programs and marketing policy. The aims of the plan are to increase the economic value of the catch and to protect the professional fishery. Fisheries statistics, catch per unit effort data and other material were collected during 1972–1976. The total allowable catch (TAC) for the most important fish species was estimated with MSY and population analysis models. The results of fish stockings were studied by tagging and population analysis calculations. Fishing of vendace and non-valuable species (perch, roach, smelt) can be increased, but the fishing pressure on other species should not be raised above the present level. Restrictions on whitefish fishing are needed in some areas. A balanced multispecies fishery is desirable, and suggestions are given for the composition of the fishing gear. Fish stocking can make possible a larger and more valuable catch, but at present its profitability is rather low. The stocking results are strongly affected by the fishery and the gear composition.  相似文献   

5.
Seasonal changes in the diet of two abundant notothenioid fishes, rock cod Patagonotothen ramsayi (Nototheniidae) and frogmouth Cottoperca gobio (Bovichthidae) were studied on the Falkland Island shelf. The rock cod is a near-bottom browser, feeding mostly on gelatinous and crustacean plankton. The diet of juveniles [10–16 cm total length (TL)] includes mostly copepods in summer and comb jellies Mnemiopsis leydyi in winter. Adult rock cod (17–34 cm TL) in summer consumed mainly plankton, with comb jellies and salps being of primary importance. In winter they mainly took benthos, primarily ophiuroids and lobster krill ( Munida spp.), and the comb jelly. During the fishing season, rock cod also scavenge on fishing discards. The frogmouth is a benthic ambush predator; both small (11–30 cm) and large (31–80 cm) representatives feed on the rock cod throughout the year. During the seasonal offshore migration of the squid Loligo gahi (June–August), this prey predominated in the large frogmouth diet, with crab, Peltarion spinosulum , of secondary importance. The region is seasonally exploited by the squid trawl fishery. As the fishing season commences (August–September), discarded rock cod also becomes an important food item for the frogmouth. Seasonal offshore migrations of L. gahi into the common depth range of both notothenioids have a significant direct (change in the C. gobio diet) and indirect (scavenging on fishing discards and targeting bottom scavengers by P. ramsayi and C. gobio during the squid fishing) impact on their diets over the Falkland shelf. The niche breadth of rock cod varied from 2.63 to 3.21 and that of frogmouth varied from 2 to 6.29, depending on fish size and season.  相似文献   

6.
In the present study, we examined the hypothesis that stretch of tendinous tissue in the human tibialis anterior (TA) muscle-tendon unit upon isometric dorsiflexion maximum voluntary contraction (MVC) varies along the entire tendinous component length. Ultrasound-based measurements of the excursions of the TA tendon origin and proximal end of the TA central aponeurosis were taken in the transition from rest to MVC in six men. Subtracting the TA tendon origin excursion from the excursion of the aponeurosis proximal end, the aponeurosis excursion was estimated. Estimation of the aponeurosis proximal region excursion was obtained subtracting the excursion of the insertion point of a central region fascicle on the aponeurosis from the whole aponeurosis excursion. Subtracting tendon excursion from the excursion of the central fascicle insertion point, the aponeurosis distal region excursion was estimated. Strain values were calculated dividing the excursions obtained by the original resting lengths. All excursions and lengths were measured in the mid-longitudinal axis of the TA muscle-tendon unit at the neutral anatomical ankle position. Tendon excursion and strain were 0.5+/-0. 08 cm (mean+/-SE) and 3.1+/-0.2%, respectively. Aponeurosis excursion and strain were 1.1+/-0.15 cm and 6.5+/-0.6%, respectively. Aponeurosis distal region excursion and strain were 0.3+/-0.05 cm and 3.5+/-0.3%, respectively. Aponeurosis proximal region excursion and strain were 0.8+/-0.12 cm and 9.2+/-1%, respectively. Aponeurosis excursion and strain were larger by 110-120% (P<0.05) compared with tendon. Aponeurosis proximal region excursion and strain were larger by 165-170% (P<0.05) compared with aponeurosis distal region. These findings are in line with results from in vitro animal material testing and have important implications for theoretical models of muscle function.  相似文献   

7.
The ability to accurately estimate the age of fishes is critical for conducting stock assessments and developing fishery management policies. Scales were collected from albacore, Thunnus alalunga, caught in the Mediterranean Sea during the years 1989–1995 to estimate their age and growth. Ages, which ranged from 1+ to 6+ years, were estimated from the interpretation of the concentric rings on the scales of 473 individuals that ranged in fork length from 55.5 to 89 cm. Males reached a greater size and age than females. The relatively close agreement in the mean lengths at ages estimated by scales and other techniques constituted a preliminary verification of the method. The von Bertalanffy growth model was fitted to mean lengths at estimated ages, resulting in the following growth parameters for the combined sexes: L = 86 cm, K = 0.4, to = ?0.8 years. Parameter estimates were in agreement with what is known about life history of the species in the Mediterranean. Moreover, the growth rates were consistent with length increment observations from five tag returns, which lend support to our working hypothesis that the scale‐rings are annual structures. When the Mediterranean albacore growth parameters were compared with those of Atlantic Ocean albacore using scale age estimates, there were significant differences between the two populations, and Mediterranean albacore remain significantly smaller than Atlantic Ocean albacore.  相似文献   

8.
Ontogenetic changes in temperature preference of Atlantic cod   总被引:4,自引:0,他引:4  
Final thermal preferendum ( T ) experiments were conducted in a horizontal thermal gradient tank from the beginning of August 2001 to mid‐November 2001 using Atlantic cod Gadus morhua from 6·5 to 79·0 cm fork length ( L F). The value of T varied significantly ( P  < 0·005) with L F( T  = 7·23–0·054 L F), with smaller (younger) fish choosing higher temperatures than larger (older) fish. The preferendum varied from 6·9° C for fish of 6·5 cm to 3·0° C for those of 79·0 cm. Experiments comparing fish positions in the gradient tank between thermal gradients of 0·5–11·0 and 4·5–14·5° C demonstrated that fish positions were determined by temperature selection instead of undesirable tank effects. This study is the first to demonstrate the effect of ontogeny on temperature preferences of a marine fish species.  相似文献   

9.
A study was conducted in 2000 and 2003, following the collapse of the commercial fishery in 1990, to compare metazoan parasites of Atlantic cod Gadus morhua, captured off coastal Labrador, with samples taken in 1980 and 1986. Fish were captured by otter trawl offshore in the North Atlantic Fish Organisation subarea 2J. Parasites were removed from the digestive tract, stained, identified and compared between the different groups. Both the prevalence and mean abundance of trematodes, larval nematodes and E. gadi were significantly lower in fish taken in 2000 and 2003 than in 1980. While mean values of trematodes and nematodes declined in 1986, those of Echinorhynchus gadi remained unchanged in 1986 and 1990. Four-year-old cod sampled in 1990 harboured significantly fewer E. gadi than older age groups. The most commonly occurring trematodes included Podocotyle reflexa, Lepidapedon elongatum, Derogenes varicus and Hemiurus levinseni while the larval nematode, Anisakis sp. was predominant. Comparison of offshore samples taken in 2000 and 2003 with others taken in previous years suggests an overall decline of parasites coincident with a change in climatic conditions, the absence of a major food source, namely capelin Mallotus villosus, of cod and ultimately the decline of the Labrador population.  相似文献   

10.
Acoustic tags and receivers were used to investigate the spatial ecology of coastal Atlantic cod Gadus morhua (n = 32, mean fork length: 50 cm, range: 33–80 cm) on the Norwegian Skagerrak coast in 2012. Monthly home ranges (HR), swimming activity and depth use varied considerably among individuals and through the months of June, July and August. HR sizes for the period ranged from 0·25 to 5·20 km2 (mean = 2·30 km2). Two thirds of the tagged G. morhua were infected with black spot disease Cryptocotyle lingua parasites; these fish had larger HRs and occupied deeper water compared with non‐infected fish. The infected fish also tended to be more active in terms of horizontal swimming. From an ecological and evolutionary perspective, any environmental change that modifies G. morhua behaviour may therefore also alter the parasite load of the population, and its conservation and fishery status.  相似文献   

11.
Recent warming has caused a northern extension in the distribution of many southern fish species in Icelandic waters. Polar cod (Boreogadus saida) around Iceland are near the southern limit of their distribution, but are poorly studied in the area. Therefore, material sampled during demersal fish surveys in March 1985–2013 and in pelagic 0-group surveys in Iceland–East Greenland waters in August–September 1974–2003 was used to investigate their distribution, abundance and biology. Demersal polar cod were most often caught on the outer shelf to the north-west and north of Iceland, but during years of widest distribution and highest abundance, they were caught farther to the east on the northern shelf. Pelagic 0-group polar cod were only caught sporadically and mainly confined to the waters off the north-west shelf of Iceland and the East Greenland shelf (southern Denmark Strait). In demersal hauls, the number of stations with polar cod decreased with increasing bottom temperature and polar cod were most widely distributed in the years 1989, 1994 and 1995. Highest numbers of demersal polar cod per haul were caught at temperatures of ?0.5 to 2.5 °C and at 200–450 m depth. The length of demersal polar cod ranged from 5 to 32 cm, while the fish caught in the pelagic trawl ranged from 2.2 to 19 cm. The polar cod in the subarctic waters north of Iceland most likely originate from the waters off East Greenland and further warming and decline in sea ice may eventually lead to the disappearance of polar cod from Icelandic waters.  相似文献   

12.
Shrimp trawling fishery in the Gulf of California captures a wide variety of non-target species of fish, crustaceans, and mollusks that are relatively unknown. The Pacific flagfin mojarra Eucinostomus currani is a frequently found species in these catches, nevertheless, nothing is currently known about its population dynamics. To contribute to the knowledge on this fish species, we studied the size structure, growth, mortality, and the recruitment pattern during the 2004-2005 seasons. A total of 6,078 mojarra were captured from 350 samples, with minimum and maximum lengths of 4.5 cm and a maximum of 21.0 cm. The average total length of the four major cohorts was 11.4, 13.7, 15.6 and 18.0 cm, corresponding to ages 0.9, 1.2, 1.6 and 2.2 years, respectively, being the most abundant the 1.2 year-old group. The instant growth coefficient indicated moderate growth rates (K(s) = 0.81/year, K(E) = 0.85/year), corresponding to individuals living between 3.5 to 3.7 years. The estimated asymptotic lengths was L (infinity) = 21.8 cm. In general, the population could be considered healthy: natural mortality (M = 1.53/year); total mortality (Z = 2.73/year); condition factor (K = 0.01072); fishery mortality (F = 1.2/year) and exploitation rate (E = 0.43/year). The maximum reproduction period almost coincided with the closed season for shrimp fishing (March to August), thus we concluded that survival of the species is ensured because reproduction is indirectly protected.  相似文献   

13.
The worldwide depletion of major fish stocks through intensive industrial fishing is thought to have profoundly altered the trophic structure of marine ecosystems. Here we assess changes in the trophic structure of the English Channel marine ecosystem using a 90-year time-series (1920–2010) of commercial fishery landings. Our analysis was based on estimates of the mean trophic level (mTL) of annual landings and the Fishing-in-Balance index (FiB). Food webs of the Channel ecosystem have been altered, as shown by a significant decline in the mTL of fishery landings whilst increases in the FiB index suggest increased fishing effort and fishery expansion. Large, high trophic level species (e.g. spurdog, cod, ling) have been increasingly replaced by smaller, low trophic level fish (e.g. small spotted catsharks) and invertebrates (e.g. scallops, crabs and lobster). Declining trophic levels in fisheries catches have occurred worldwide, with fish catches progressively being replaced by invertebrates. We argue that a network of fisheries closures would help rebalance the trophic status of the Channel and allow regeneration of marine ecosystems.  相似文献   

14.
From 1994 to 2003, a total of 526 bitches of 99 different breeds were artificially inseminated in 685 estrus cycles with domestic (n = 353) or imported (n = 332) frozen-thawed semen from 368 males. The overall whelping rate was 73.1% and mean (+/- S.E.M.) litter size 5.7 +/- 0.1 pups. The whelping rate was higher after intrauterine insemination (75.0%; n = 665) than after intravaginal insemination (10.0%, n = 20; P < 0.05). Insemination at the optimal time resulted in a higher whelping rate (78%, n = 559; P < 0.01) and larger litter size (5.8 +/- 0.2; P < 0.05) than inseminations performed late or too late (55.7% and 4.5 +/- 0.5, n = 61). Two inseminations (n = 384) yielded a higher whelping rate (P < 0.05) and mean litter size (P < 0.01) than one insemination (n = 241), 78.1% and 6.0 +/- 0.2 and 70.5% and 5.1 +/- 0.2, respectively. For inseminations performed at the optimal time, however, the whelping rate was not significantly different for bitches inseminated twice (79.3%, n = 358) versus once (76.8%, n = 168), but the litter size was larger (6.0 +/- 0.2 and 5.3 +/- 0.3). Semen classified as of poor quality (progressive motility < 50% or percentage abnormal sperm > 20%) resulted in a lower whelping rate (P < 0.01) than semen classified as of good quality (progressive motility > or = 50% and percentage abnormal sperm < or = 20%), 61 and 77%, respectively. Small breeds (n = 50) had a smaller litter size (3.9 +/- 0.3; P < 0.01) than larger breeds (medium [5.7 +/- 0.3, n = 94], large [5.9 +/- 0.2, n = 295] or giant breeds [6.1 +/- 0.5, n = 62] [P < 0.01]). Bitches older than 6 years had a lower whelping rate (68.2%) than younger ones (77.0%; P < 0.05). The duration of pregnancy was longer (P < 0.01) for bitches with a litter size of < 3 pups (61.7 +/- 0. 4 days, n = 30) than for bitches with larger litters (60.5 +/- 0.1 days, n = 177). These results show the potential of transcervical intrauterine insemination for routine artificial insemination in dogs. The results with frozen semen inseminations were optimised by inseminating bitches < or = 6 years old 2 and 3 days after ovulation with semen of good quality from males < or = 8 years old.  相似文献   

15.
1. Determinations of retinal, total lipid and lipid phosphorus were made on 10ml. samples of cod plasma. 2. Immature control fish, injected with 0.2ml. of carrier oil/kg. wt., had 0.73+/-0.12mug. of retinal/100ml. of plasma, and maturing male fish had similar concentrations. Maturing female fish had about 10mug. of retinal/100ml. of plasma. 3. Immature male or female cod given single intramuscular injections of 1mg. of oestradiol-17beta 3-benzoate in 0.2ml. of oil/kg. wt. had 8.54+/-0.59 mug. of retinal/100ml. of plasma after 5 days and about 25mug./100ml. of plasma after 10 days. 4. Oestradiol injections had little effect on the concentration of plasma phospholipids, and no effect on lipids other than phospholipids. 5. For all 116 fish examined, regardless of sex or treatment, the concentration of plasma phospholipid was significantly correlated with that of lipids other than phospholipids (r=0.727), and phospholipids formed 50.4% of the total lipids in cod plasma. 6. Alcohol dehydrogenase was purified from cod liver and shown to oxidize retinol to retinal. It was completely inhibited by 0.1mm-oestradiol. Alternative modes of action of oestradiol are discussed.  相似文献   

16.
Pacific saury Cololabis saira were found in 24 out of 1314 stomachs of demersal fishes collected by bottom trawling at 245–393 m depths. The same area was occupied by the stick-held dipnet fishery for saury, in which a landing limitation was enforced to avoid overfishing. The fish species containing saury were: Pacific cod Gadus macrocephalus , walleye pollock Theragra chalcogramma , and oilfish Ruvettus pretiosus . These demersal species would not encounter living saury naturally, which normally are distributed above thermoclines. The ingested saury were smaller than market size which suggests that the saury from fish stomachs were discarded by fishermen because of small sizes to maximize profits under the landing limitation.
The scavenged saury made up 41.5, 24.6 and 77.7% of the diets of large-sized (>30cm)Pacific cod, walleye pollock and oilfish, respectively in terms of DW composition. The extent to which discarded saury contributed to the total diet in the demersal fish assemblage, calculated by considering species composition and diets of bottom fishes, was 21.8% of the total diet. The discarded saury seemed to compensate the less productive feeding environment during autumn for the bottom fishes.  相似文献   

17.
During fish disease surveys for marine rhabdoviruses in 1993 and 1995, the cod ulcus syndrome was seen widely in all ages of cod, especially the 2 to 5+ year classes. Viral Haemorrhagic Septicaemia Virus (VHSV) was isolated from a small proportion of the lesion-positive fish and these isolates were identified by immunofluorescence or ELISA. A serendipitous observation of dermal petechiae on haddock was made. VHSV was isolated from this lesion for the first time indicating a new host species for VHSV.  相似文献   

18.
Trophic indicators were used to compare two Malian freshwater reservoirs whose main differences are based on their different fishing pressures. Data were collected from a scientific survey of small-scale fishery landings conducted in 2002/2003. The trophic levels of fish species caught by artisanal fisheries are estimated from observations of scientific fishing or from the metabase Fishbase. Important differences exist in the trophic structure of both reservoirs. In Selingue (with high fishing pressure), very few top predators are found in the catches while the low trophic level fishes increase in total catches. In Manantali (with low fishing pressure), the top predators contribute twice as much to catches compared to Selingue. Hence, the mean trophic level of catches in Selingue (2.80) is lower than in Manantali (2.97). When comparing these results with those of study made in 1994/1995, it clearly appears that the effects of the fishing pressure in Selingue are obvious through a decrease of 0.12 in the mean trophic level while in Manantali this mean level has increased by 0.33 due to a recent strategic targeting of top predators. Trophic spectra seem to be relevant tools to characterize exploited fish communities from multi-specific and multi-gear small-scale fisheries catch data.  相似文献   

19.
This paper examines the life history of humpback red snapper Lutjanus gibbus, an important fishery species for coastal communities across the Indo-Pacific, in southern New Caledonia, where the species is lightly exploited. A total of 243 L. gibbus were sampled between January 2013 and December 2016 from occasional harvests of commercial fishers. Examination of sectioned otoliths revealed that opaque increment formation occurred annually between November and March, coinciding with the species' spawning season. Estimates of maximum age were similar between sexes, with observed ages of 38 and 36 years for females and males, respectively, extending the reported longevity of this species by at least 11 years. Growth differed significantly between sexes, with males reaching greater length at age and greater asymptotic length than females (38.88 v. 31.46 cm fork length (LF). Total mortality for all samples was estimated as 0.13 and was slightly higher for males (0.16) than females (0.11). Estimates of natural and fishing mortality were low and slightly higher for males than females. Male L. gibbus were found to mature at slightly greater lengths and younger ages than females, with the length and age at which 50% of individuals attained maturity estimated to be 25.8 cm LF and 3.9 years of age for females and 26.8 cm LF and 3.4 years of age for males. The results provide key baseline information from which to assess the effect of fishing on the species for populations in New Caledonia and adjacent locations and, when viewed with those of other studies, highlight the importance of understanding spatial patterns in demography of harvested fish species across gradients of exploitation and environmental influences.  相似文献   

20.
Diet investigations were carried out on 0-, 1- and 2-year-old Northeast Arctic cod (Gadus morhua) sampled in the Barents Sea during 1984–2002. Stomach-content analyses showed that the 0 and 1 group cod fed mainly on crustaceans, with krill and amphipods composing up to 70% of their diet. Krill (Thysanoessa spp. and Meganyctiphanes norvegica) and amphipods (Themisto spp.) were mainly found in cod stomachs sampled in the central and close to the Polar Front region in the Barents Sea where these prey organisms are reported to be abundant in summer. A shift in the main diet from crustaceans to fish was observed from age 1 to age 2. The diet of 2-year-old cod mainly comprised capelin (Mallotus villosus) and other fish, and to a lesser degree, krill and amphipods. Shrimp (mainly Pandalus spp.) was also an important prey in both age 1 and 2 cod. A statistically significant positive relationship was obtained between capelin stock size and the amount of capelin in the diet of 2-year-old cod. Results from this study also show that the larger age-2 cod preyed more on capelin in winter and that larger cod (>22 cm) prefer larger capelin (>12 cm). During periods of low capelin abundance, the 2-year-old cod shift their diet more to crustaceans, such as krill and amphipods. A positive significant relationship was also obtained between Total Fullness Index (TFI) and the amount of capelin in the diet and between TFI and the growth of 2-year-old cod, indicating that the growth of age-2 cod is to a large extent dependent on the amount of capelin consumed. Growth of age-1 cod was also positively correlated to TFI.  相似文献   

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