Temperature preference of juvenile lumpfish (Cyclopterus lumpus) originating from the southern and northern parts of Norway

Fish are ectothermic animals and have body temperatures close to that of the water they inhabit. They can still control their body temperatures by selecting habitats with temperatures that maximize their growth, feed conversion and wellbeing. Lumpfish, Cyclopterus lumpus, is widely distributed in the North Atlantic Ocean and therefore exposed to variable water temperatures. Lumpfish is extensively used as cleanerfish in salmon farming in Norway and exposed to a wide temperature range along the north-south axis of the Norwegian coastline. But, if these temperature ranges correspond to the preference temperatures of lumpfish is not known. If lumpfish has adapted to regional temperatures along the Norwegian coast, differences in preference temperature for fish from different regions should be evident. In a selective breeding perspective, different selection lines for preference temperature would then be useful for further development of lumpfish as a cleanerfish.We subjected lumpfish juveniles weighing 154–426g originated from northern (Group North – GN) and southern (Group South – GS) Norway to a temperature preference test, using an electronic shuttle box system. The system allowed the fish to control the water temperature by moving between two chambers, and thereby choosing its preferred temperature in the range from 5 to 16 °C. We started the temperature at 7.8 ± 1.37 °C for GN and 7.58 ± 1.34 °C for GS, but all the fish except four (two each from GN and GS) chose lower temperatures (5.03–7.6 °C) in the first 18 h and stayed closer to that temperature during the next 30 h. Based on the results, GN and GS lumpfish preferred 6.92 ± 1.8 and 6.2 ± 1.2, respectively, and there was no significant difference between the groups. Neither was there any significant difference in growth rates (SGR) between the two groups. Based on our results, we suggest that lumpfish from any geographical origin along the Norwegian coast can be used anywhere in Norway. It follows that lumpfish from a single selection line could be used at any salmon farm in Norway independent of its location.     

Notes on the behaviour of lumpfish in sea pens with and without Atlantic salmon present

The behaviour of lumpfish, Cyclopterus lumpus L., in sea pens, with and without Atlantic salmon, Salmo salar L., present, was assessed by underwater camera technology. Behaviour was classified by recording the principal activity of individual fish for 30-s intervals. The majority of daylight time was spent actively foraging for food. Antagonistic behaviour between Atlantic salmon and lumpfish was not observed during the whole experimental period and no mortality was seen in either species. Cleaning behaviour, but at low frequency, was observed as lumpfish cleaned sea lice off Atlantic salmon. Significantly lower sea lice infection levels were seen on Atlantic salmon when reared together with lumpfish compared to the control group without lumpfish. Feeding behaviour can be classified as strongly opportunistic.     

Seasonal occurrence of sea lice Lepeophtheirus salmonis on sea trout in two north Norwegian fjords

Seasonal occurrence of the parasitic copepod Lepeophtheirus salmonis (sea lice) was studied from March to December 2001 in two large north Norwegian sill fjords without fish farming activity, the Ranafjord and the Balsfjord. Anadromous brown trout Salmo trutta (sea trout) in both fjords had a low infestation rate during all sampling periods, but followed a seasonal pattern. During early and late winter (November to December and March to April) and spring (May to June), the prevalence varied from 0 to 25% and the abundance was <0·5 sea lice. Adults dominated (92%) during this period, particularly gravid females. In both fjords, the highest prevalence was during September (80–81%, all stages represented). In Ranafjord, the abundance and mean intensity during this month was 6·8 and 8·6 sea lice, respectively, while in Balsfjord it was 3·6 and 4·5 sea lice, respectively. Fish were captured at temperatures down to 1° C and at full strength sea water which is supposed to cause osmoregulatory problems for the fish. This observation has implications for the understanding of high‐latitude sea trout behaviour and can also make the fish more vulnerable to heavy sea lice infestation during this period. It is suggested that winter running sea trout help to maintain a self replicating local population of sea lice within such fjord systems where other possible hosts ( e.g . farmed Atlantic salmon Salmo salar ) are not present during a whole year cycle.     

Commercial trials using emamectin benzoate to control sea lice Lepeophtheirus salmonis infestations in Atlantic salmon Salmo salar

Two trials were conducted at commercial salmon farms to evaluate the efficacy of emamectin benzoate (Slice, 0.2% aquaculture pre-mix, Schering-Plough Animal Health) as a treatment for sea lice Lepeophtheirus salmonis (Kr?yer) and Caligus elongatus Nordmann infestations in Atlantic salmon Salmo salar L. Trials were carried out in 15 m2 commercial sea pens, at temperatures of 5.5 to 7.5 degrees C and 10.8 to 13.8 degrees C. Each pen was stocked with 14,000 to 17,500 fish with mean weights of 0.44 to 0.74 and 1.33 to 1.83 kg. Fish were naturally infested with sea lice at the start of each trial. At Day -1, samples of 10 or 15 fish were taken from each pen to determine pre-treatment numbers of lice. Emamectin benzoate was administered in feed, to 4 replicate pens, at a dose of 50 micrograms kg-1 biomass d-1 for 7 consecutive days (Days 0 to 6). Sea lice were counted again, between Days 7 and 77, and comparisons made with untreated control fish. Despite adverse weather conditions, wide variations in fish weights and exposure to new infestations, treatment was effective against chalimus and motile stages of L. salmonis. In the autumn trial, efficacy at Day 27 was 89%, and lice numbers remained lower on treated fish than on control fish 64 d from the start of treatment. In the winter trial, reductions in lice numbers at low temperatures were slower but good efficacy was achieved by Day 35. Although control fish had to be treated with hydrogen peroxide at Day 21, fish treated only with emamectin benzoate on Days 0 to 6 still had 89% fewer lice than control fish at Day 35. There were very few C. elongatus present, but at the end of both trials numbers were lower on treated fish. No adverse effects were associated with treatment of fish with emamectin benzoate.     

Helminth communities of the lesser sandeel Ammodytes tobianus L. off the west coast of Ireland

The helminth parasites present in 412 lesser sandeels (Ammodytes tobianus) taken from June 1996 to May 1997 from the Aran Islands on the west coast of Ireland were examined. Ten helminth parasite species were recorded, and more than 92% of the sandeels were infected with at least 1 helminth species. Seven of the species were digeneans, including Brachyphallus crenatus, Hemiurus communis, Derogenes varicus, Lecithaster gibbosus, Opechona bacillaris, Cryptocotyle lingua, and Galactosomum lacteum; 2 nematodes, including Hysterothylacium sp. and Contracaecum sp.; and 1 cestode, Scolex pleuronectis. Three of the 7 digenean species were either larvae or immature. Only 2 species, the digeneans G. lacteum and H. communis, had prevalences greater than 50%. The dominant species was G. lacteum, accounting for 67% of all parasites present. The relationship between spawned groups, host length, and season versus the abundance, prevalence, species richness, and the total number of parasites in the infracommunities was investigated. No difference was found between the parasite communities of the 2 spawning races of the host population. Mean abundance and prevalence of the different parasite species showed seasonal variation. Numbers of parasite species and numbers of parasites increased with fish length. The role of A. tobianus as an intermediate host for helminths was assessed; it was determined that most were infectious to birds or mammals, with the majority of the parasite species being autogenic (infectious to fish). The mean number of parasites per fish was nearly a quarter of the value recorded for A. tobianus in the North Sea, where a much higher intensity of infection was recorded.     

Species identification of small fish in Xixuan Island coastal waters of Zhoushan using DNA barcoding

This study aimed to assess the fish species composition in Xixuan Island coastal waters and confirm the applicability of DNA barcoding in fish identification. Fish samples were caught with a gill net in December 2016 and in January, February, April, May and June 2017, and the fish species in these samples were analyzed and identified via both traditional morphological methods and DNA barcoding (mitochondrial 12S rRNA). The sample contained 1,357 individuals, the majority of which were Tridentiger barbatus (829 individuals). A total of 154 individuals were selected for DNA barcode analysis, and 147 sequences were obtained. A total of 35 species, which belonged to six orders and 16 families, were accurately identified. One species was identified to the genus level (Repomucenus), and only one was identified to the family level (Moridae). Most of the species belonged to Gobiidae (12 species). With the exception of those in Moridae, the identified species were commonly detected on the Zhejiang coast. The average Kimura two-parameter (K2P) distances within species, genera, families, and order and between orders were 0.15%, 4.44%, 14.96%, 20.37%, and 26.20%, respectively. The intraspecific K2P distance was markedly greater than the interspecific distance. The A + T content (55.41%) was higher than the G + C content (44.58%). Phylogenetic tree analysis showed that individuals belonging to the same species were clustered together and could be clearly distinguished. Mitochondrial 12S rRNA can be effectively used for juvenile and adult fish identification and has good application prospects in fish species identification. The investigation and species identification of small fish collected monthly from Xixuan Island coastal waters are conducive to understanding the distribution characteristics and species composition of fish in the Zhoushan coastal area.     

Genetic Structure of Capelin (Mallotus villosus) in the Northwest Atlantic Ocean

Capelin (Mallotus villosus) is a commercially exploited, key forage-fish species found in the boreal waters of the North Pacific and North Atlantic Oceans. We examined the population structure of capelin throughout their range in the Canadian northwest Atlantic Ocean using genetic-based methods. Capelin collected at ten beach and five demersal spawning locations over the period 2002 through 2008 (N = 3,433 fish) were genotyped using six polymorphic microsatellite loci. Temporally distinct samples were identified at three beach spawning locations: Chance Cove, Little Lawn and Straitsview, Newfoundland. Four capelin stocks are assumed for fisheries management in the northwest Atlantic Ocean based on meristics, morphometrics, tag returns, and seasonal distribution patterns. Our results suggested groupings that were somewhat different than the assumed structure, and indicate at least seven genetically defined populations arising from two ancestral populations. The spatial mosaic of capelin from each of the two basal cluster groups explains much of the observed geographic variability amongst neighbouring samples. The genetic-defined populations were resolved at Jost’s D _est ≥ 0.01 and were composed of fish collected 1) in the Gulf of St. Lawrence, 2) along the south and east coasts of Newfoundland, 3) along coastal northern Newfoundland and southern Labrador, 4) along coastal northern Labrador, 5) near the Saguenay River, and at two nearshore demersal spawning sites, 6) one at Grebes Nest off Bellevue Beach on the east coast of Newfoundland, and 7) one off the coast of Labrador at Domino Run. Moreover, the offshore demersal spawners on the Scotian Shelf and Southeast Shoal appeared to be related to the inshore demersal spawners at Grebes Nest and in Domino Run and to beach spawners from the Gulf of St. Lawrence.     

Migration of Pacific Rim Chum Salmon on the High Seas: Insights from Genetic Data

Wild stocks of chum salmon, Oncorhynchus keta, have experienced recent declines in some areas of their range. Also, the release of hatchery chum salmon has escalated to nearly three billion fish annually. The decline of wild stocks and the unknown effects of hatchery fish combined with the uncertainty of future production caused by global climate change have renewed interest in the migratory patterns of chum salmon on the high seas. We studied the composition of high-seas mixtures of maturing and immature individuals using baseline data for 20 allozyme loci from 356 populations from throughout the Pacific Rim. Composition estimates were made from three time series. Two of these time series were from important coastal migratory corridors: the Shumagin Islands south of the Alaska Peninsula and the east coast of the Kamchatka Peninsula. The third was from chum salmon captured incidentally in the Bering Sea trawl fishery for walleye pollock. We also analyzed geographically dispersed collections of chum salmon captured in the month of July. The time series show dynamic changes in stock composition. The Shumagin Island corridor was used primarily by Northwest Alaskan and Asian populations in June; by the end of July stocks from the Alaska Peninsula and southern North America dominated the composition. The composition along the Kamchatka coast changed dramatically from primarily Russian stocks in May to primarily Japanese stocks in August; the previously undocumented presence of stocks from the Alaska Peninsula and Gulf of Alaska was also demonstrated. Immature chum salmon from throughout the Pacific Rim, including large proportions of southern North American stocks, contributed to the Bering Sea bycatch during the months of September and October. The migration routes of North American stocks is far more widespread than previously observed, and the Bering Sea is an important rearing area for maturing and immature chum salmon from throughout the species' range.     

Infestations of wild adult Atlantic salmon (Salmo salar L.) by the ectoparasitic copepod sea louse Lepeophtheirus salmonis Krøyer: prevalence,intensity and the spatial distribution of males and [2pt] females on the host fish

The copepod Lepeophtheirus salmonis Krøyer is a specific ectoparasite of North Atlantic and Pacific salmonids in their marine phases. We compared infestations of L. salmonis on wild Atlantic salmon (Salmo salarL.) captured in estuarine (Firth of Tay, east Scotland; 1995, 1996) and marine coastal waters (Strathy Point, north Scotland; 1998, 1999). Host fish from the Tay were caught by sweep netting, whilst those from Strathy Point were trapped in anchored bagnets. Fish capture method and exposure of the parasites to brackish conditions may both have detrimental effects on the retention of L. salmonis by the host, and hence possibly lead to their being under-estimated on returning adult fish. At Strathy Point, we recorded (i) an infestation prevalence of 100%, (ii) mean log abundances of pre-adult + adult L. salmonis at 19 (1998) and 24 (1999) per fish, (iii) 85/93% of all L. salmonis as being adults and (iv) overall 68/69% females. Fish caught in the upper Firth of Tay showed significantly lower prevalences, intensities and abundances of L. salmonis and probably had lost part or all of their lice burdens prior to capture, whereas those sampled from Strathy Point were apparently minimally affected by capture method or brackish water influences. The loss of parasites for the Tay fish was not markedly biased to males or females, or to pre-adult versus adult developmental stages. There were significantly greater abundances of L. salmonis on two sea-winter fish (30 lice per fish) than on one sea-winter fish (17 lice per fish) sampled at Strathy Point in 1998. There are several possible explanations for such age-related patterns of abundance, but the indications are that (i) initial infestation of smolts occurs in coastal waters, (ii) infestation of hosts in the open ocean is a persistent event, and (iii) oceanic reinfestation outweighs mortality losses of L. salmonis. This parasite typically occupies rather few zones on the host fish covering only a small percentage of the total available body surface area. Female predominance appears to be characteristic of L. salmonisinfestations of wild Atlantic salmon; this is in marked contrast to reports of extreme male dominance on farmed stocks. Adult females predominated on the epidermis adjacent to, and posterior of, the insertion of the anal fin and along the posterior dorsal midline between the dorsal and caudal fins. Males predominated on the sides of the head and along the dorsal midline between the head and the dorsal fin. Mate guarding/precopulatory pairs are formed between pre-adult II females and adult males. The significant correlation between the distribution of pre-adult females and adult males may be indicative of pre-adult females actively seeking out adult males, but more likely is due to the (large) adult females competitively ousting all smaller life stages (female and male) from those preferred zones. Given the relatively low fecundity of adult females, and observations of 100% prevalence of L. salmonis, the infective planktonic copepodid stage evidently is extremely efficient at locating and establishing upon its host fish in the pelagic environment.     

Results of satellite tagging of Atlantic bluefin tuna, <Emphasis Type="Italic">Thunnus thynnus</Emphasis>, off the coast of Ireland

Pop-up satellite archival tags were attached to six Atlantic bluefin tuna (Thunnus thynnus) off the west coast of Ireland in autumn 2003 and 2004. The satellite tags measured pressure, ambient temperature and light for the term of deployment. Radio pop-up satellite endpoint positions, light and sea surface temperature estimations of geolocation indicate that two fish tagged minutes apart off the coast of County Donegal, migrated to the eastern and western Atlantic Ocean over the following 8 months. The two fish were 5218 km apart at the termination of the experiment. After tagging in September and popping up the following March and April, one fish had traveled to the western Atlantic while the other was located in the waters off the southwest coast of Portugal. A third fish tagged off the coast of County Donegal in October 2004 moved into the Mediterranean Sea and was caught by a fishing vessel southeast of Malta on 11 June 2005. The results link bluefin tuna feeding on European foraging grounds with known eastern breeding regions and western Atlantic waters.     

Salmon lice increase the age of returning Atlantic salmon

The global increase in the production of domestic farmed fish in open net pens has created concerns about the resilience of wild populations owing to shifts in host–parasite systems in coastal ecosystems. However, little is known about the effects of increased parasite abundance on life-history traits in wild fish populations. Here, we report the results of two separate studies in which 379 779 hatchery-reared Atlantic salmon smolts were treated (or not) against salmon lice, marked and released. Adults were later recaptured, and we specifically tested whether the age distribution of the returning spawners was affected by the treatment. The estimates of parasite-induced mortality were 31.9% and 0.6% in the River Vosso and River Dale stock experiments, respectively. Age of returning salmon was on average higher in treated versus untreated fish. The percentages of fish returning after one winter at sea were 37.5% and 29.9% for the treated and untreated groups, respectively. We conclude that salmon lice increase the age of returning salmon, either by affecting their age at maturity or by disproportionately increasing mortality in fish that mature early.     

The occurrence of Lepeophtheirus salmonis and Caligus clemensi (Copepoda: Caligidae) on three-spine stickleback Gasterosteus aculeatus in coastal British Columbia

Infections with sea lice species belonging to Lepeophtheirus and Caligus are reported from examinations of 1,309 three-spine sticklebacks collected in coastal British Columbia. Over 97% of the 19,960 Lepeophtheirus specimens and nearly 96% of the 2,340 Caligus specimens were in the copepodid and chalimus developmental stages. The parasites were identified as Lepeophtheirus salmonis and Caligus clemensi based on morphology of adult stages. Between 1,763 and 1,766 base pairs (bp) of 18S rDNA from adult specimens collected from sticklebacks and salmon differed from the GenBank L. salmonis reference sequence by a single bp and were distinct from those of 2 other Lepeophtheirus species. A 530-bp region of 18S rDNA from chalimus stages of Lepeophtheirus obtained from sticklebacks and salmon was identical to that of the L. salmonis reference sequence. The three-spine stickleback is a new host record for L. salmonis. The prevalence of L. salmonis was 83.6% and that of C. clemensi was 42.8%. The intensities of these infections were 18.3 and 4.2, respectively. There was no significant relationship between sea lice abundance and stickleback condition factor. Significant spatial and temporal variations both in abundance of sea lice and surface seawater salinities were measured. The abundance of both sea lice species was lowest in zones in which surface seawater salinity was also lowest. Sticklebacks appear to serve as temporary hosts, suggesting a role of this host in the epizootiology of L. salmonis. The stickleback may be a useful sentinel species with which to monitor spatial and temporal changes in the abundance of L. salmonis and C. clemensi.     

Spatial and temporal variations in parasite prevalence and infracommunity structure in herring (Clupea harengus L.) caught to the west of the British Isles and in the North and Baltic Seas: implications for fisheries science

Herring Clupea harengus L. viscera were examined for endoparasitic infections as part of a multidisciplinary stock identification project (WESTHER, EU Contract no. Q5RS-2002-01 056) which applied a range of stock discrimination techniques to the same individual fishes to obtain comparable results for multivariate analysis. Spawning and non-spawning adults, and juvenile herring were caught, over 3 years, by commercial and research vessels from numerous locations to the west of the UK and Ireland, along with control samples of spawning fish from the eastern Baltic Sea, and juveniles from sites in the eastern and western North Sea, and the north of Norway. The metacercariae of two renicolid digeneans (Cercaria pythionike and Cercaria doricha), one larval nematode (Anisakis simplex s.s.) and one larval cestode (Lacistorhynchus tenuis) were selected as tag species. Results were compared with those from herring collected between 1973 and 1982, which suggested remarkable stability in the parasite fauna of herring in the study area. These species were used to compare the parasite infracommunities of spawning herring. A significant variation in infracommunity structure was observed between different spawning grounds. These results suggest that the parasite fauna of herring are spatially variable but remain temporally stable in both the short and long term. Significant differences in prevalence and abundance of infections and comparisons of parasite infracommunity enabled the separation of putative herring stocks west of the British Isles. Distinctive patterns of parasite infection in two different spawning groups off the north coast of Scotland suggest that this area is occupied by two spawning populations, one recruiting from the west of Scotland, the other from outside this area, and most likely from the eastern North Sea. The distribution patterns of L. tenuis, C. doricha and C. pythionike suggest the potential for fish that spawn in three distinct International Council for the Exploration of the Seas (ICES) management units to be present in mixed aggregations found over the Malin Shelf, with significant implications for management in this area.     

The diversity of sea lice (Copepoda: Caligidae) parasitic on threespine stickleback (Gasterosteus aculeatus) in coastal British Columbia

The prevalence, intensity, and abundance of sea lice belonging to Lepeophtheirus or Caligus clemensi are reported from threespine stickleback (Gasterosteus aculeatus) collected from the Broughton Archipelago region of coastal British Columbia, Canada, between 2005 and 2008. In total, 25,130 sea lice were collected from 7,684 sticklebacks. The prevalence of Lepeophtheirus ranged from 51% in 2005 to 11% in 2008 and that of C. clemensi from 56% in 2007 to 24% in 2008. Chalimus stages accounted for approximately 69% of all Lepeophtheirus and 88% of Caligus specimens. Cytochrome c oxidase subunit 1 (COI) gene sequences, useful in distinguishing reference specimens belonging to 8 species of Lepeophtheirus, Caligus, and Bomolochus, were used to identify the Lepeophtheirus specimens from stickleback as L. salmonis (71%) and L. cuneifer (29%). A COI phylogenetic analysis confirmed a monophylogenetic origin of Lepeophtheirus but not of Caligus. Two genotypes were resolved in L. cuneifer, i.e., genotype A occurred twice as often as genotype B. Virtually all immature Lepeophtheirus specimens from juvenile salmon were L. salmonis. The results emphasized the need to accurately identify immature sea lice as a prerequisite to understanding sea lice ecology. The threespine stickleback may be a useful sentinel species for the abundance and diversity of the sea lice that are also parasites of wild and farmed salmon in coastal ecosystems in British Columbia.     

Early life history of the small yellow croaker (Larimichthys polyactis) in sandy ridges of the South Yellow Sea

The early life history of the small yellow croaker Larimichthys polyactis was studied using the otolith microchemistry of juvenile fish and a recruitment survey in the sandy ridges of the South Yellow Sea. The otolith Sr/Ca ratio regime shifted markedly at the age of 5–7 days (early April) and 37–41 days (around early May). Subsequently, the Sr/Ca ratio was almost constant until late June, indicating an aggregation of L. polyactis juveniles in the relatively stable habitat of the sandy ridges, which was supported by a recruitment survey that showed a rapid increase in the number of juveniles in the same area between early May and the end of June. Variations in the Sr/Ca ratio between early April and early May might reflect a habitat adaptive strategy of L. polyactis in the pelagic stage to hydrodynamic processes of the sandy ridges. Based on the aforementioned habitat shifts and recruitment phenology, it was hypothesised that L. polyactis juveniles disperse between early April and early May, inhabit sandy ridges to feed, and migrate offshore in late June.     

Spatial and temporal variations in sea lice (Copepoda: Caligidae) infestations of three salmonid species farmed in net pens in southern Chile

Sea lice infestations have become a major health problem for farmed salmonids throughout the world including Chile. In southern Chile, 6 geographical areas, divided into 22 geographical zones with a total of 127 salmon farming centers and 1519 sea pens, were regularly sampled from December 1999 to April 2002. A linear mixed-effects model (LME) approach was used to describe the infestations of adult forms of sea lice on 3 salmonid species farmed in southern Chile. The variables fish species, water temperature, water salinity, fish weight, juvenile parasite count, pen shape, treatment status in previous month and the interaction of previous and current month treatments were found to be statistically significant fixed effects for the population sampled. The most susceptible species to sea lice infestation was rainbow trout Oncorhynchus mykiss, while the least susceptible species was coho salmon O. kisutch. Fishes in pens treated in the previous month with avermectins were associated with the smallest sea lice count compared to fishes in pens not treated or treated with other products. The variability in sea lice infestations in areas and zones within areas was not statistically significant when controlling for the previously mentioned fixed variables. The variability between centers, the within-pen variability, and the interaction between within-pen effect and the date of measurement were statistically significant and not explained by the fixed effects. Potential sources for this variability are discussed. We conclude that the epidemiology of sea lice infestations in farmed salmonids in southern Chile is complex and in need of further study.     

Ovarian cycling of tilefish, Lopholatilus chamaeleonticeps Goode and Bean, from the South Atlantic Bight, U.S.A.

Reproductive cycling and oocyte development were examined for female tilefish, Lopholatilus chamaeleonticeps Goode and Bean, captured in the South Atlantic Bight, U.S.A. A total of 752 female tilefish was collected during 15 sampling periods from May 1982 to December 1983. Reproductive cycling was quantified using four techniques: gross visual estimation of ovarian stages, histological examination of ovaries, follicle diameter measurements, and an ovarian index. The relative accuracies of these techniques also were evaluated. All four methods indicated that L. chamaeleonticeps was capable of spawning from March to June. Most spawning occurred between April and June. Ovarian development was positively correlated with the change in daylength. Follicular development was asynchronous, which is characteristic of multiple spawners. Folliculogenesis was similar to that in most oviparous teleosts.     

黄海山东海域春、秋季鱼类群落多样性

根据2006年春季(5月)和秋季(10月)黄海山东海域疏目变水层双拖网调查数据,作者对该海域鱼类群落多样性、空间分布及长度谱特征进行了研究。调查共捕获鱼类61种,其中鲈形目种类最多(27种),其次为鲱形目(8种)和鲉形目(7种);按摄食类型可分为浮游动物食性、浮游动物/底栖动物食性、底栖动物食性、底栖动物/游泳动物食性、游泳动物食性、杂食性、碎屑食性7个类群,其中浮游动物食性占渔获物重量组成的82.5%。春、秋季鱼类群落均主要由小型种类占优势,春季优势种有3种,为玉筋鱼(Ammodytes personatus)、鳀(Engraulis japonicus)和方氏锦鳚(Pholis fangi),秋季优势种有5种,分别是鳀、青鳞小沙丁鱼(Sardinella zunasi)、玉筋鱼、小黄鱼(Larimichthys polyactis)和赤鼻棱鳀(Thryssa kammalensis)。丰富度指数(D)、Shannon-Wiener多样性指数(H’)、均匀度指数(J’)的空间异质性明显,表现出近岸海域(尤其是青岛-乳山南部近岸海域)较高,深水区较低的态势,但多样性指数的季节间差异不显著。资源量与Shannon-Wiener多样性指数(H’)、均匀度指数(J’)、功能团多样性指数(FD)均呈负相关关系,与种类丰富度指数(D)则无相关关系。春、秋季鱼类群落长度谱曲线斜率均小于-0.2,而截距均大于10,与该海域1985年及2000年的研究结果相比,长度谱斜率显著降低、截距显著增加,这与小型个体种类占优势密切相关。与历史资料相比,黄海山东海域鱼类群落多样性下降,小型个体比例上升,资源呈衰退趋势。