Timecourse of oocyte development in saithe Pollachius virens

Wild caught North Sea saithe Pollachius virens were monitored for growth, sex steroid profiles and oocyte development pre‐spawning and measured for egg size and group fecundity during the spawning season in the laboratory. Vitellogenesis commenced in late October–early November, at a leading cohort size (C_L) of c. 250 µm, after which oocytes grew rapidly in size until spawning started in February. Notably, a distinct cortical alveoli stage was virtually absent with yolk granules observed in developing oocytes at the very beginning of vitellogenesis. Little atresia was observed pre‐spawning, but atretic re‐absorption of remnant oocytes containing yolk granules was found in all females immediately post‐spawning. As expected, concentrations of sex steroids, oestradiol‐17β (females), testosterone (both sexes) and 11‐ketotestosterone (both sexes), increased pre‐spawning before dropping post‐spawning. The present experiment provides the first validation of sex steroid levels in P. virens. Post‐ovulatory follicles were visible in histological sections from female gonads 9–11 months post‐spawning, but then disappeared. Spawning commenced around a C_L of c. 750 µm (700–800 µm). Hydrated oocytes (eggs) measured between 1·04 and 1·31 mm (mean = 1·18 mm) with decreasing sizes towards the end of spawning. The average estimated realized fecundity was c. 0·84 million eggs (median female total length, L_T = 60 cm). Spawning lasted from 13 February to 29 March.     

Empirical standard mass equation for Salmo marmoratus

Total length (L_T) (range 24–1000 mm; mean ±s.e . = 170·21 ± 0·36 mm) and mass (W) (range 0·10–9590 g; mean ±s.e . = 76·03 ± 0·87 g) of 36 460 specimens of marble trout Salmo marmoratus were used to compute a standard mass (W_s) equation for this species by means of the empirical percentile (EmP) method. The EmP W_s equation calculated was: log₁₀W_s = ?5·208 + 3·202 log₁₀L_T? 0·046 (log₁₀L_T)² (L_T range 90–570 mm) and it is valid throughout the species' area of distribution across Europe.     

Captive hybridization of two geographically isolated pygmy angelfish species,Centropyge fisheri and Centropyge resplendens

This study documents the rearing of two pygmy angelfish species, Centropyge fisheri and Centropyge resplendens, and the early life history and reproduction of their hybrid offspring. A C. fisheri female, collected from Hawaii, and a C. resplendens male, captive‐bred from parental stock collected from Ascension Island, were maintained at the hatchery facility for 7 months. Continuous spawning was achieved at a photoperiod cycle of 14L:10D and a water temperature of 26·5° C, range ±1° C. Over the 110 day period, the C. fisheri female spawned 102 times, 57% of which resulted in embryos (fertilized eggs). The mean ±s.d. fecundity per spawn was 730 ± 459 eggs (range 52–1967). Fertility (% eggs that developed into embryos) of all eggs that were preserved was 22·4 ± 25·6%. A total of 235 hybrid juveniles were raised through metamorphosis with an average larval survival of 16·4%. Eight F₁ hybrid juveniles isolated for further study began to display signs of reproductive behaviour c. 300 days post‐hatch (dph). Spawn resulting in non‐fertile eggs were first obtained 319 dph, and fertilized eggs developing into embryos were obtained after 411 dph from at least two female individuals. While no attempt was made at rearing the F₂ larvae, embryo and larval development were normal up to 8 dph. Reproduction and development observed for all hybrid generations in this study were normal, similar to other Centropyge species and indicates a very close phylogenetic relationship between what are currently considered distinct species, e.g. C. fisheri and C. resplendens.     

Resource partitioning in two carcinophagous confamiliars: distribution and feeding habits of psychrolutid fish on the southern Patagonian shelf

Psychrolutes marmoratus and Cottunculus granulosus, two psychrolutids, inhabit the shelf and continental slope of the southern South America between 45 and 1,250 m. Over this range, the P. marmoratus is found in depths of less than 400 m, whilst C. granulosus inhabits waters of >200 m. There is a trend of increasing total length with increasing water depth for P. marmoratus and the converse for C. granulosus. The total lengths of the species are different in the depth zone that they overlap in (200–400 m), and we hypothesise that this may reduce interspecific competition. Both are specialised carcinophagous feeding on crabs, large isopods and sea spiders, but they also prey upon polychaets and gastropods. There was no difference in the feeding spectra of similar-sized fish sampled at the same depth, indicating a similar foraging strategy. In contrast to this, the feeding spectra are quite different between the species over different depths as well as at different adult sizes. The feeding niche breadth in these species is similar to other slope dwelling fish in the area.     

Life history and reproductive ecology of the endangered Itasenpara bitterling Acheilognathus longipinnis (Cyprinidae) in the Himi region,central Japan

The life history, reproductive ecology and habitat utilization of the Itasenpara (deepbody) bitterling Acheilognathus longipinnis were investigated in a lowland segment of the Moo River in Toyama Prefecture, central Honshu, Japan. Analysis of 1285 individuals revealed that the study population comprised a single size class, an age at maturation of 3 months and a life span of 1 year. On the basis of the growth pattern, the life cycle was divided into two stages: the juvenile stage, characterized by rapid growth, and the adult stage at which growth ceased. Spawning by A. longipinnis was recorded between early September and late October. Female A. longipinnis in the 0+ year age class began to mature when they reached a standard length (L_S) of 56·4 mm. Mature females had a large clutch size (maximum 273 eggs) and deposited highly adhesive and relatively large eggs (2·55 mm³; major axis, 3·12 mm; minor axis, 1·22 mm) via a short ovipositor (mean length, 21·5 mm) into freshwater mussels. The embryos remained in the gill cavities of the freshwater mussels (used as a spawning substratum) and emerged as juveniles (L_S, 9 mm). Habitat utilization during spawning was analysed using a generalized linear model. The best‐fit model showed that three environmental factors (freshwater mussel availability, water depth and vegetation cover) were important variables for habitat utilization by A. longipinnis. Shallow areas (water depth, 250–330 mm) created for rice paddy management and areas with an abundance of cover were particularly effective for predator avoidance. These results suggest that maintenance of water level fluctuations corresponding with rice cultivation and the abundance of vegetation on the river bank (particularly avoidance of concrete revetments) is essential for conservation of this species under current practices for rice cultivation in Japan.     

Reproductive biology of albacore Thunnus alalunga

Reproductive variables in albacore Thunnus alalunga were evaluated by gonad histology in samples of 132 males (58–118 cm fork length, L_F) and 112 females (59–101 cm L_F) that were collected from the western North Pacific Ocean from 2001 to 2006. In the sex ratio examination, males greatly outnumbered females in large adult fish (L_F > 100 cm). Thunnus alalunga exhibited a protracted spawning period from March to September in the waters off eastern Taiwan and the Philippines, and the peak spawning activity occurred in March and April. Minimum sizes associated with the classification of mature fish were 78 and 83 cm L_F for males and females, respectively. In addition, the largest L_F of immature fish were 93 cm for males and 94 cm for females. The spawning frequency estimate in April was 1·7 days. Batch‐fecundity estimates of 21 females (89–99 cm L_F) ranged between 0·17 and 1·66 million eggs (mean ±s.d . = 0·94 ± 0·43). The relative fecundity estimates of the 21 females ranged between 9·2 and 92·4 oocytes g^?1 body mass (mean ±s.d . = 50·5 ± 22·8). The results presented in this study provide increased information regarding this species' reproductive‐related characteristics than are currently available in stock status determinations.     

Development of the cottid fish,Pseudoblennius percoides,reared in the laboratory,with brief descriptions of juvenileP. marmoratus andP. zonostigma

Embryonic, larval and juvenile development of the cottid fish,Pseudoblennius percoides were described on the basis of a series of laboratory-reared specimens. The eggs were demersal, adhesive, almost spherical in shape, measuring 1.66–1.82 mm in diameter, and with numerous various-sized oil globules. Neighboring eggs adhered to each other to form an egg mass. Hatching occurred between 13 and 16 days after spawning at a water temperature of 15.4 to I6.5°C. Newly hatched larvae measured from 6.5 to 7.3 mm, averaging 6.9 mm TL, and possessed 40 myomeres. Absorption of the yolk was completed at about 7.5 mm TL. Flexion of the notochord started and finished at about l0 mm TL and about 14 mm TL, respectively. Aggregate numbers of all fin rays were completed at over 16 mm TL, when the larvae reached the juvenile stage. The pigment pattern became the same as that of adults in juveniles longer than 25 mm TL. Lateral lines were completed at over 44 mm TL, when the juveniles attained to the young stage. The early stages of this species were clearly distinguished from those ofP. cottoides, and the juveniles of fourPseudoblennius species, i.e.P. percoides, P. cottoides, P. marmoratus andP. zonostigma, could be identified mainly by their pigment patterns.     

Protracted volitional spawning of pinfish Lagodon rhomboides and changes in egg quality and fatty‐acid composition throughout the spawning season

Spawning performance of pinfish Lagodon rhomboides without use of hormonal aids was monitored over an extended season. Nearly three million eggs were obtained from 75 spawns collected over a 90‐day consecutive period from a single population of four brood fish (1M:1F). A mean ± s.d. batch fecundity of 30·27 ± 22·64 eggs g^?1 female was estimated with 98·0 ± 0·06% of the batch composed of floating eggs which were 1·04 ± 0·04 mm in diameter and 85·71 ± 27·59% fertile. Floating eggs successfully hatched 54·65 ± 29·13% of the time which yielded larvae that were 2·59 ± 0·24 mm in length. Fatty acids within floating eggs were largely represented by polyunsaturated fatty acids (45·30 ± 2·14% of total fatty acids) of which linoleic acid [(c18:2n‐6cis) 3·49 ± 1·69% trifluoroacetic acid (TFA)] and docosahexaenoic acid (DHA) [(c22:6n‐3) 28·47 ± 1·48% TFA] represented the majority of fatty acids for n‐6 and n‐3 polyunsaturated fatty acids, respectively. The strongest correlations between fatty acids and hatching success and larval survival to first feeding were observed for the DHA:EPA (eicosapentaenoic acid; c20:5n‐3) ratio and total n‐6 polyunsaturated fatty‐acids levels, respectively. These data demonstrate potential for producers to rely on natural spawns for extensive egg production and provide a baseline for future development of natural spawning protocols of captive L. rhomboides.     

Zygoparity and sex steroid hormone profiles in bluemouth Helicolenus dactylopterus

Two hundred and seven individuals (103 females and 104 males) of bluemouth Helicolenus dactylopterus (Scorpaeniformes, Sebastidae), a commercially important deep‐water species with an unusual reproductive strategy, from the eastern Atlantic Ocean ranging from 13·9 to 37·5 cm total length (L_T) were analysed from September 2011 to October 2012. The analysis included gonad maturity phases and blood‐plasma levels of oestradiol‐17β (E₂), 11‐ketotestosterone (11‐KT) and 17,20β‐dihydroxypregn‐4‐en‐3‐one (17,20β‐P). Results confirmed the existence of an annual reproductive cycle with asynchrony between females and males and a spawning season from January to May. A pronounced peak in 17,20β‐P in October for both sexes was associated with possible mating behaviour and recent copula. Levels of E₂ increased preceding the elevation of the gonado‐somatic index during ovarian growth and were lower during regression and regeneration. The frequency distribution of oocyte–embryonic stages and variation of hormone levels suggest the existence of daily rhythms. Fertilization was detected between 2000–0000 and 0800–1200 h and spawning took place throughout the day peaking between 2000 and 0000 h. The cyclic pattern of sex steroids and ovarian recruitment provides a new insight into the reproductive strategy of this species.     

Elevated water temperature impairs fertilization and embryonic development of whitefish Coregonus lavaretus

The adverse effects of high temperatures on the early life stages of anadromous whitefish Coregonus lavaretus were experimentally examined by assessing fertilization success, the percentage of developmental abnormalities, cumulative mortality and the rate of embryogenesis across a range of temperatures. Temperatures ≥ 7° C increased the proportion of unfertilized and abnormally dividing eggs, deformed embryos and consequent mortality. The higher the temperature, the more severe were the effects. When eggs were fertilized and constantly incubated at various temperatures, the effective level for 50% of the eggs and embryos (EL50) of temperature was 7·6° C at the developmental stage when eye pigmentation was visible. Fewer developmental abnormalities and a lower cumulative mortality rate were observed when embryos were exposed to high temperatures from the later, gastrula stage, than from fertilization or the four‐cell stage. Irrespective of retarded development in terms of day‐degrees (i.e. the sum of daily mean temperatures), a high incubation temperature reduced the development time of C. lavaretus, leading to earlier hatching, and hatched fry were shorter than at the reference temperature of 4–5° C. Global warming will particularly pose risks for stenothermic species such as C. lavaretus, with early life stages being especially susceptible. Thus, relatively small increases and fluctuations in river water temperatures during the spawning season of this anadromous species may have substantial negative impacts on its recruitment and population persistence.     

Life history of a simultaneously hermaphroditic fish,Diplectrum formosum

Sand perch Diplectrum formosum were collected in the Atlantic Ocean waters off the south‐eastern U.S. for life history analyses between April 2001 and July 2004 and ranged in size from 63 to 236 mm standard length (L_S) and 0–8 years of age. Diplectrum formosum are simultaneous hermaphrodites that reach 50% sexual maturity at 12·2 months and 122·8 mm L_S for testicular tissue and 13·6 months and 129·3 mm L_S for ovarian tissue. The gonad contains ovarian and testicular tissue separated by a thin basement membrane, with no means of internal self‐fertilization. Spawning females were found between March and January with a peak of spawning activity in May and a realized spawning periodicity of 2 days, equivalent to a maximum of 168 spawning events per year. Fish with testicular tissue in spawning condition were obtained throughout the year in every month sampled with a maximum frequency between June and September. There were also trends indicating that testicular tissue was more likely to be in spawning condition in the presence of hydrated oocytes within the accessory structure. This may indicate the use of the accessory structure as a storage site for hydrated oocytes until a mate can be located.     

Timing of paddlefish spawning in the Upper Missouri River,Montana, USA in relation to river conditions

Spawning activity of paddlefish Polyodon spathula in the Missouri River, Montana in 2008–2009 was examined to delineate spawning sites and times in relation to discharge, water temperature and turbidity. One hundred thirty‐six eggs were collected at water temperatures ranging from 12.0 to 20.7°C (mean, 16.3°C; SD, 2.5). Only 12 of 89 (13%) congregations of radio‐tagged adults observed during the spawning period coincided with egg captures. Six larvae were collected at water temperatures ranging from 19.1 to 21.7°C (mean, 20.5°C; SD, 0.86). Peak discharge in 2008 (903 m³ s^?1 on 14 June) was approximately 30% greater in magnitude and occurred 11 days later than peak discharge in 2009 (612 m³ s^?1 on 3 June). Despite these differences in the hydrograph, no significant differences in egg CPUE were found between years (anova , F = 0.69, P = 0.56). Logistic regression identified no significant river condition variables associated with the presence or absence of eggs (P > 0.14 for all variables). However, in both years maximum egg CPUE was recorded within 3 days of the hydrograph peak and at similar water temperatures (17.5°C in 2008, 16.8°C in 2009). These results suggest an overall association of peaking discharge and seasonally warming water temperatures with egg deposition. Higher catches of eggs and larvae than observed in this study may be necessary to clarify short‐term (day‐to‐day) effects of environmental changes on spawning activity. Continued investigation of the relationship between short‐term changes in river conditions and paddlefish spawning activity is needed to understand the mechanics underlying the reproductive success of this species.     

Reproductive biology and spatiotemporal patterns of spawning in striped marlin Kajikia audax

This study presents the first histology‐based assessment of the reproductive dynamics of south‐west Pacific striped marlin Kajikia audax. Maturity and reproductive status were assessed from histological sections of ovaries (n = 234) and testes (n = 243) of fish caught in commercial longline and recreational fisheries between 2006 and 2009. Spawning peaked in the Coral Sea during November and December at sea surface temperatures between 24·8 and 28·3° C. Lower jaw fork length (L_LJF) at 50% maturity (L_LJF50), a key variable for stock assessment, was estimated to be 2100 ± 102 mm (mean + s.e .) for females and 1668 ± 18 mm for males. Unlike large pelagic tunas Thunnus spp., the proportion of females increased with length and spawning fish formed multiple large‐scale aggregations within a broad latitudinal band. This study provides a starting point for biological parameters needed for stock assessment and conservation of K. audax and introduces the multiple aggregation spawning concept as a reproductive mechanism to explain genetic heterogeneity observed in some highly migratory species.     

Pollination activity by blowflies and honeybees on onions in breeders' cages

Honeybees (Apis mellifera) and three species of blowflies (Calliphora vomitoria, Lucilia caesar and L. sericata) were observed on mass-pollinated populations of onions (Allium cepa) in 4·4 × 3·6 m × 2-2·5 m cages in June-August 1977. Pollination activity at temperatures from 14° to 28°C was compared on the basis of mean time per flower touched on each umbel visit. Honeybees did not forage below 16°C. Above 16°C their mean time per flower was short (1·4 s) and varied little with temperature. For blowflies, it decreased markedly from 12·1 s at 14–15·5°C to 2·7 s at 26°C and above, largely because at low temperatures flies spent long periods quiescent or grooming rather than actively feeding. When flowering was not completely synchronous between cultivars, honeybees were more selective than blowflies, but where flowering was synchronous, both types of pollinator visited the two cultivars at random. For the size of cage used, neither type of insect had a distinct advantage as a pollinator of onions, despite their different behaviour patterns.     

Life‐history traits of the long‐nosed skate Dipturus oxyrinchus

This work investigates life‐history traits of the long‐nosed skate Dipturus oxyrinchus, which is a common by‐catch in Sardinian waters. The reproductive variables were analysed from 979 specimens sampled during scientific and commercial hauls. Females (10·4–117·5 cm total length, L_T) attained larger sizes than males (14·5–99·5 cm L_T). To evaluate age and growth, a sub‐sample of 130 individuals (76 females and 54 males) were used. The age was estimated by annuli counts of sectioned vertebral centra. Four models were used for the length‐at‐age data: the von Bertalanffy, the exponential, the Gompertz and the logistic functions. According to the Akaike's information criterion, the Gompertz model seemed to provide the best fitting curve (L_∞ mean ± s.e. : 127·55 ± 4·90 cm, k: 0·14 ± 0·09, IP: 3·97 ± 0·90 years). The oldest female and male were aged 17 (115·5 cm L_T) and 15 years (96·0 cm L_T), respectively. Lengths at maturity were 103·5 cm for females and 91·0 cm for males, corresponding to 90% of the maximum observed length in both sexes. The monthly distribution of maturity stages highlighted an extended reproductive cycle, with spawning females and active males being present almost throughout the year, as confirmed by the gonado‐somatic index. Ovarian fecundity reached a maximum of 26 yolked follicles with a mean ± s.e. size of 19·7 ± 6·5 mm.     

The annual marine feeding aggregation of Atlantic sturgeon Acipenser oxyrinchus in the inner Bay of Fundy: population characteristics and movement

Atlantic sturgeon Acipenser oxyrinchus aggregate to feed from May to October in Minas Basin (45° N; 64° W), a large, cul‐de‐sac embayment of the inner Bay of Fundy. The aggregation consists mainly of migrants from the Saint John, NB and Kennebec Rivers, ME (99%). During 2004–2015, 4393 A. oxyrinchus were taken as by‐catch by commercial fish trawlers or at intertidal fishing weirs, and 1453 were marked and/or sampled and released. Fork length (L_F) ranged from 458 to 2670 mm, but 72·5% were <1500 mm. Mass (M) ranged from 0·5 to 58·0 kg. The mass‐length relationship for fish ≤50 kg was log₁₀M = 3·32log₁₀L_F ? 5·71. Observed growth of unsexed A. oxyrinchus recaptured after 1–8 years indicated fish of 90–179 cm L_F grew c. 2–4 cm a year. Ages obtained from pectoral spines were from 4 to 54 years. The Von Bertalanffy growth model predicted K = 0·01 and L_∞ = 5209 mm L_F. Estimated annual mortality was 9·5–10·9%. Aggregation sizes in 2008 and 2013 were 8804 and 9244 individuals, respectively. Fish exhibited high fidelity for yearly return to Minas Basin and population estimates indicated the total at‐sea number utilizing the Basin increased from c. 10 700 in 2010 to c. 37 500 in 2015. Abundance in the Basin was greatest along the north shore in spring and along the south shore in summer, suggesting clockwise movement following the residual current structure. Marked individuals were recaptured in other bays of the inner Bay of Fundy, north to Gaspé, Quebec, and south to New Jersey, U.S.A., with 26 recoveries from the Saint John River, NB, spawning run. Fish marked at other Canadian and U.S. sites were also recovered in Minas Basin. Since all A. oxyrinchus migrate into and out of the Basin annually they will be at risk of mortality if planned tidal power turbines are installed in Minas Passage.     

Induced ovulation and embryonic development of ocellated puffer, Takifugu ocellatus

Acute injections of different hormones to induce ovulation in mature ocellated puffer, Takifugu ocellatus, collected from natural waters during the spawning season, were carried out to develop a reliable protocol for mass production of seed in this species. All experimental fish were divided into seven groups treated with: a saline injection (control), single or two injections of luteinizing hormone‐releasing hormone analog (LHRH‐a; single injection: 50 μg kg^?1, two injections: 10 and 40 μg kg^?1), single or two injections of pituitary (single injection: 6 mg kg^?1, two injections: 1 and 5 mg kg^?1) and single or two injections of human chorionic gonadotropin (hCG; single injection: 2500 IU kg^?1, two injections: 500 and 2000 IU kg^?1), respectively. The percentage of fish that ovulated in six hormonal treatments reached 100%, either with a single injection or with two injections whereas the fish in control group failed to spawn. There were no significant differences among all hormonal treatments in egg production, fertilization rate, or hatch rate (P > 0.05) except time to ovulation between a single injection group and the two‐injection group (P < 0.05). The fertilized eggs of ocellated puffer were spherical, demersal, and adhesive. They had a mean oocyte diameter of 1.487 ± 0.106 mm (range: 1.404–1.560). The egg membrane was transparent and yolk was buff in color, containing a cluster of small oil globules. Thirty‐four successive stages of embryonic development were identified and characterized. Fertilized eggs incubated at 18–20°C generally commenced hatching at 144 h after fertilization. Newly hatched larvae were about 3.26–3.45 mm in length. The induced ovulation technique using acute injections of hormones is an important step in the development of the culture of the ocellated puffer.     

Reproductive parameters of rhinobatid and urolophid batoids taken as by‐catch in the Queensland (Australia) east coast otter‐trawl fishery

Reproductive variables are provided for batoids regularly taken as by‐catch in the east coast otter‐trawl fishery on the inner‐mid continental shelf off the south‐east and central coasts of Queensland, Australia. Total length at maturity (L_T50 and 95% c.i .) for the eastern shovelnose ray Aptychotrema rostrata was 639·5 mm (617·6–663·4 mm) for females and 597·3 mm (551·4–648·6 mm) for males. Litter size (n = 9) ranged from nine to 20 (mean ± s.e. = 15·1 ± 1·2). This species exhibited a positive litter size–maternal size relationship. Disc width at maturity (W_D50 and 95% c.i .) for the common stingaree Trygonoptera testacea was 162·7 mm (155·8–168·5 mm) for females and 145·9 mm (140·2–150·2 mm) for males. Gravid T. testacea (n = 6) each carried a single egg in the one functional (left) uterus. Disc width at maturity (W_D50 and 95% c.i .) for the Kapala stingaree Urolophus kapalensis was 153·7 mm (145·1–160·4 mm) for females and 155·2 mm (149·1–159·1 mm) for males. Gravid U. kapalensis (n = 16) each carried a single egg or embryo in the one functional (left) uterus. A single female yellowback stingaree Urolophus sufflavus carried an embryo in each uterus. A global review of the litter sizes of shovelnose rays (Rhinobatidae) and stingarees (Urolophidae) is provided.     

Aquatic food‐web dynamics following incorporation of nutrients derived from Atlantic anadromous fishes

Changes in the isotopic composition (δ¹³C and δ¹⁵N) in biofilm, macro‐invertebrates and resident salmonids were used to characterize temporal dynamics of marine derived nutrients (MDNs) incorporation between stream reaches with and without MDN inputs. Five Atlantic rivers were chosen to represent contrasting MDN subsidies: four rivers with considerable numbers of anadromous fishes; one river with little MDN input. Rainbow smelt Osmerus mordax, alewife Alosa pseudoharengus, sea lamprey Petromyzon marinus and Atlantic salmon Salmo salar, were the primary anadromous species for the sampled rivers. Regardless of the spatial resolution or the pathway of incorporation, annual nutrient pulses from spawning anadromous fishes had a positive effect on isotopic enrichment at all trophic levels (biofilm, 1·2–5·4‰; macro‐invertebrates, 0·0–6·8‰; fish, 1·2–2·6‰). Community‐wide niche space shifted toward the marine‐nutrient source, but the total ecological niche space did not always increase with MDN inputs. The time‐integrated marine‐nutrient resource contribution to the diet of S. salar parr and brook trout Salvelinus fontinalis ranged between 16·3 and 36·0% during anadromous fish‐spawning periods. The high degree of spatio‐temporal heterogeneity in marine‐nutrient subsidies from anadromous fishes lead to both direct and indirect pathways of MDN incorporation into stream food webs. This suggests that organisms at many trophic levels derive a substantial proportion of their energy from marine resources when present. The current trend of declining anadromous fish populations means fewer nutrient‐rich marine subsidies being delivered to rivers, diminishing the ability to sustain elevated riverine productivity.     

Myotube production in fast myotomal muscle is switched-off at shorter body lengths in male than female Atlantic halibut Hippoglossus hippoglossus (L.) resulting in a lower final fibre number

A sampling method is described to determine accurately the number of fast myotomal muscle fibres (N_F) in a large flatfish species, the Atlantic halibut Hippoglossus hippoglossus. An unusual feature of the fast myotomal muscle is the presence of internalized strips of slow muscle fibres. In fish of 1·5–3·5 kg (n = 24), the total cross‐sectional area (A_TC) of fast muscle was 18% greater in the dorsal than ventral myotomal compartments (P < 0·05), whereas there was no significant difference between left‐ and right‐hand sides of the body. Due the bilateral asymmetry, muscle blocks (5 × 5 × 5 mm) were prepared to systematically sample each myotomal quadrant (dorsal, ventral, left‐ and right‐side) and the diameters of 150 fast fibres measured per block. Smooth non‐parametric probability functions were fitted to a minimum of 800 measurements of fibre diameter per quadrant (n = 5). There were no significant differences in the distribution of muscle fibre diameters between myotomal compartments and therefore N_F could be estimated from a single quadrant. The number of blocks required to estimate N_F with a repeatability of ±2·5% increased from six at 300 g body mass to 17 at 96·5 kg, caused by variation within and between blocks. Gompertz curves were fitted to measurements of fibre number and fork length (L_F). The estimated final fibre number was 8·96 × 10⁵ (7·99–9·94 × 10⁵, 95% CI) for males and 1·73 × 10⁶ (1·56–1·90 × 10⁶, 95% CI) for female fish. The estimated L_F for cessation of fibre recruitment in the fast muscle of female fish (1775 mm) was almost twice that in males (810 mm), reflecting their greater ultimate body size.