Taxonomy of the deep-sea eel genus, Histiobranchus (Synaphobranchidae, Anguilliformes), with notes on the ecology of H. bathybius in the eastern North Atlantic

The eel genus Histiobranchus Gill occurs benthopelagically over the continental rise and abyss of the World Ocean, primarily beneath temperate and subpolar surface waters. Its generic status within the subfamily Synaphobranchinae is confirmed by comparison of the structure and topography of its cephalic sensory system and skeletal features with Synaphobranchus. At least three species of Histiobranchus are recognized: H. bathybius (panoceanic), H. bruuni (Tasman Sea and waters south-east of New Zealand) and H. australis (two geographical forms; South Atlantic and south-western Indian Ocean, and South Indo-West Pacific Oceans). Collections totalling 319 specimens of H. bathybius from the eastern North Atlantic (1790–5440 m depth) yielded a size range of 99–1370 mm total length ( L _T), with no apparent sexual dimorphism. Length–frequency distributions indicate a mode of juvenile fish at around 100–200 mm L _T and a further two around 600–700 and 1300–1400 mm L _T among adults. Generally smaller fish occur in shallower regions, although the size range is broad over the whole depth range. No apparent trend occurs in the size distribution with latitude over the range 17–54) N. Females outnumber males (1 male : 1·7 females) and both sexes are largely distinguishable from 300 mm L _T. Ripening eggs occur in females from both adult length modes, with running ripe and spent females of very different size indicating iteroparity.     

Substratum selection by juvenile flounder Platichthys flesus (L.): effect of ephemeral filamentous macroalgae

Selection of substrata representing habitats with or without filamentous vegetation by juvenile flounder Platichthys flesus was investigated in laboratory experiments and a field study to assess the effect of the increasing occurrence of ephemeral filamentous macroalgae in shallow nursery areas and its potential effect on flounder recruitment. In the laboratory experiments, 79% of juvenile flounder (24–99 mm total length, L _T) preferred bare sand substrata in comparison to substrata with simulated filamentous vegetation. Substratum preference did not change with fish size. Field sampling using a drop trap (1 m²) in a vegetated (>50% coverage of ephemeral macroalgae) habitat in June, July and August showed small flounder (mean L _T= 37·3 mm) were caught more often in samples with less vegetation. In contrast, larger flounder (mean L _T= 57·2 and 79·7 mm in July and August, respectively) were more evenly distributed in relation to the distribution of vegetation. The results of this study indicate that the increase in ephemeral macroalgae observed in recent years potentially reduces the quantity and quality of suitable nursery habitats for juvenile flounder, which could have detrimental consequences to the overall recruitment of flounder in coastal areas.     

Onset of maturity and cohort composition at spawning of Baltic sprat Sprattus sprattus on the basis of otolith macrostructure analysis

Data on catch per unit effort (CPUE) from the Danish commercial fishery showed dense aggregations of sprat Sprattus sprattus born in 1997 in the main spawning area of the Baltic Sea during May 1999. After the analysis of their otolith macrostructure, these fish were found to have a shorter distance to the beginning of the first winter zone compared to fish from the same cohort caught in other months. At the same time, the number of fish with large otoliths at the period of winter zone formation tended to decrease or even disappear. The otolith size at winter zone formation ( O _W) was found to be a good predictor of sprat total length ( L _T) and mass ( M ) in age group 0 years. Changes in the otolith size distribution over time were related to variations in cohort composition depending on the L _T and M attained during the juvenile stage. Examination of the gonads suggested two life history patterns: 1) individuals achieving a larger L _T and M during the first growing season may contribute at age 1 years to the spawning stock and the fishery, whereas 2) individuals attaining smaller L _T and M contribute in larger amounts to both the spawning stock and the fishery later, when they are at age 2 years. Moreover, changes in the relationship between O _W and L _T and M were related to differences in growth trajectories for sprat born during the same spawning season. Smaller sprat as group 0 years continued growing during the second growing season delaying maturity compared to larger individuals from the same cohort.     

Ontogenetic changes in the diet of the herbivorous Scartichthys viridis in a rocky intertidal zone in central Chile

Scartichthys viridis maintains a herbivorous diet following recruitment to rocky intertidal areas, where it consumes almost exclusively macroalgae. The sheet–like green macroalgae Ulva and Enteromorpha were the main items consumed by individuals <130 mm L _T. The tough branching red macroalga Gelidium made the bulk of the gut contents of specimens >220 mm L _T, Ulva being consumed to a much lesser extent. Further, Gelidium increased in importance in the total gut contents during ontogeny. In contrast, both small (70–120 mm L _T) and medium–sized (140–210 mm L _T) S. viridis individuals preferred Ulva in the laboratory. It is suggested that the increasing consumption of Gelidium along the ontogeny of S. viridis results from the limited availability of Ulva in the field. Large S. viridis individuals possessed longer guts relative to their body length, in comparison with small individuals.     

Size-dependent predator–prey relationships between perch and their fish prey

Size-dependent interactions between piscivorous perch Perca fluviatilis (age ≥1 year) and their fish prey age 0 year perch, pikeperch Sander lucioperca and roach Rutilus rutilus in the biomanipulated Bautzen Reservoir indicated that the highest ratio of prey total length ( L _T) to predator L _T was 59%. Perch L _T and prey fish L _T were positively and linearly related. Perch L _T was strongly related with both gape width and gape height. Within the range 80–110 mm L _T, the gape height of perch exceeded gape width, while beginning at 120 mm L _T the gape width exceeded gape height. The minimum, maximum and mean prey L _T and prey body depths of all three prey species increased with increasing predator size, but the increases in mean sizes of perch and pikeperch as prey were less than that of roach. The low limit of the 'predation window' observed in this study coupled with results of previous studies on perch in the Bautzen Reservoir indicated that perch had a major impact on the population dynamics of both perch and pikeperch.     

Habitat use and trophic position determine mercury concentration in the straight fin barb Barbus paludinosus, a small fish species in Lake Awassa, Ethiopia

The diet, habitat use and mercury concentration of the small fish species, the straight fin barb Barbus paludinosus , were studied in Lake Awassa, Ethiopia, for a period of 1 year from February 2003 to January 2004. Stable isotope signatures of nitrogen and carbon in different total length ( L _T) classes were used to determine trophic positions and organic carbon sources, respectively. Barbus paludinosus mainly occupied the protected benthic habitats (littoral and profundal) of the lake. The δ¹³C values were in the range from −24 to −19‰, indicating that the carbon source for B. paludinosus was benthic, as well. Small individuals (≤ 60 mm L _T) mainly preyed upon ostracods, intermediate sizes (60–100 mm) on aquatic insects and gastropods, while a tiny cyprinodont fish Aplocheilichthys antinorii dominated the diet of large individuals (100–160 mm). The progressively increase in δ¹⁵N with increasing L _T also indicated a diet shift towards piscivory in larger individuals. The mercury concentration ranging from 0·02 to 0·74 mg kg⁻¹ wet mass (wm), was unexpectedly high in this small species, and was significantly positively related to L _T, as well as to δ¹⁵N. Some large individuals had mercury concentrations < 0·1 mg kg⁻¹ wm, and low δ¹⁵N, indicating substantial variations in diet between individuals of same size. The study suggests that other piscivorous species which include B. paludinosus in their diet may have a high mercury intake risk.     

Starving early juvenile sprat Sprattus sprattus (L.) in western Baltic coastal waters: evidence from combined field and laboratory observations in August and September 2003

Growth patterns inferred from otolith microstructure analysis were compared between sprat Sprattus sprattus early juveniles (26–42 mm total length, L _T) collected in August 2003 in shallow coastal waters of the Kiel Fjord, and sprat recruits (60–95 mm L _T) sampled in October during a pelagic trawl survey of the western Baltic Sea. At the end of August, a sudden and very rapid decline in otolith growth was observed in early juveniles but not in sprat recruits. Laboratory results indicated that the early juvenile fish were starving prior to capture. Specifically, when transferred to the laboratory, otolith growth rates immediately increased in fish provided ad libitum food rations, while otolith growth of starved fish continued to decline in the same manner observed prior to field collection. In addition, the vast majority of juvenile sprat had empty stomachs on the sampling day. Given that juveniles and recruits probably experienced similar temperature conditions, the rapid decline in juvenile growth rates presumably resulted from very poor feeding conditions in nearshore waters. Starvation during the early juvenile period has not been documented before, but may, at least in the case of Baltic sprat, comprise a density-dependent mechanism operating in coastal nursery areas in some years.     

Reproductive biology and implications for management of the orange-spotted grouper Epinephelus coioides in the southern Arabian Gulf

The reproductive biology of Epinephelus coioides was determined from the examination of 1455 individuals collected between July 2005 and June 2007 in the southern Arabian Gulf. Histological preparations of gonads indicated that males were either derived from a juvenile phase or the transition of postspawning females, confirming a diandric protogynous sexual pattern. The spawning season was well defined, occurring once a year during April and early May. Peaks in spawning occurred after the full and new moons and was completed within a single lunar cycle. The presence of mature males over the entire size and age range and the absence of inactive mature females during the spawning season suggested that the population was not constrained by sperm limitation. While specimens undergoing sexual transition were only observed in size and age ranges of 335–685 mm total length ( L _T) and 5–6 years, patterns in the proportion of males in size and age classes suggested that sex change occurred at a relatively constant rate after female maturation up to the maximum size (1002 mm L _T) and age (11 years). Relationships between reproductive output and capacity with size and age indicated that conventional regulations that equate the mean size at first capture to sexual maturation are unsuitable for the management of E. coioides . The maximum age, small size and young age at sexual maturation ( L _min= 320 mm L _T, 2 years, for females and 242 mm L _T, 1 year, for males) conflict with the general pattern for large epinepheline groupers and may be a direct result of the intensive demersal fishery in the southern Arabian Gulf.     

Sediment preferences and size-specific distribution of young-of-the-year Pacific halibut in an Alaska nursery

A combination of laboratory experiments and field surveys was used to test the hypotheses that responses to sediments change with fish size and that sediment grain-size is the predominant environmental factor affecting small-scale distribution in young-of-the-year (yoy) Pacific halibut Hippoglossus stenolepis . Laboratory tests showed that the smallest fish (31–40 mm L _T) chose fine sediments (muddy and fine sands), fish 51–70 mm had high selectivity (primarily medium sand), and the largest fish (80–150 mm) were not selective although they avoided the largest grain-sizes (pebbles and granules). Sediment preferences were correlated with size-dependent burial capabilities. Beam trawl collections were made over a 6 year period in Kachemak Bay, Alaska, to examine the distribution of yoy Pacific halibut (14–120 mm L _T) using small size classes (e.g. 10 mm intervals). Canonical correlation analysis showed that the per cent of sand in the sediment was a highly significant variable for all but one size and date combination. Catch per unit of effort (CPUE) for newly settled fish (<30 mm L _T) was highest on very fine sand, fish 41–80 mm were most abundant on fine sand, and the largest yoy fish (81–120 mm) were abundant over a range of sediments from fine sand to mud. Except for the smallest fish, Pacific halibut in the field were associated with sediments somewhat finer than predicted from the laboratory experiments; however, virtually all were captured where they could bury easily. The ability of flatfish to bury and shelter in sediment is related to fish size; consequently, habitat associations shift rapidly during the first year of life. Habitat models for yoy flatfishes should consider size-dependent shifts in capabilities and preferences.     

Influence of female age, body size and environmental conditions on annual egg production of the bullhead

A population of bullhead Cottus gobio inhabiting a mountainous river network, was examined for the influence of maternal condition (total length, L _T, and age class in years) and environmental factors (water temperature and fish density) on fecundity, egg size and net annual reproductive effort ( E _R, defined as the ratio of total egg production mass to body mass) within several stocks. Regardless of location within the drainage network, bullhead reproduction was synchronous, with only a single batch and fecundity ranging from 69 to 896 eggs per female. As with L _T fecundity increased with age, although the difference between age classes 5 and 6 years was smaller than between other age classes. Egg diameter also increased with age from age classes 2 to 4 years, but then tended to decrease for age classes 5–6 years. Multiple stepwise regression was used to examine the effect of female L _T, water temperature and fish densities on bullhead egg production for each of the four age classes (2, 3, 4 and 5–6 years) separately. When statistically significant, water temperature always had a positive effect and fish densities a negative effect, on fecundity and egg size. The L _T always had a positive effect on fecundity for all the age classes. In contrast, L _T had a positive effect on egg size and E _R only for female reaching maturity (2 years), whereas intermediate aged females (3–4 years) were insensitive and the oldest fish showed a negative response.     

Constraints of body size and swimming velocity on the ability of juvenile rainbow trout to endure periods without food

The hypothesis that body size and swimming velocity affect proximate body composition, wet mass and size‐selective mortality of fasted fish was evaluated using small (107 mm mean total length, L _T) and medium (168 mm mean L _T) juvenile rainbow trout Oncorhynchus mykiss that were sedentary or swimming ( c . 1 or 2 body length s⁻¹) and fasted for 147 days. The initial amount of energy reserves in the bodies of fish varied with L _T. Initially having less lipid mass and relatively higher mass‐specific metabolic rates caused small rainbow trout that were sedentary to die of starvation sooner and more frequently than medium‐length fish that were sedentary. Swimming at 2 body length s⁻¹ slightly increased the rate of lipid catabolism relative to 1 body length s⁻¹, but did not increase the occurrence of mortality among medium fish. Death from starvation occurred when fish had <3·2% lipid remaining in their bodies. Juvenile rainbow trout endured long periods without food, but their ability to resist death from starvation was limited by their length and initial lipid reserves.     

Gape limitation and piscine prey size‐selection by yellow perch in the extreme southern area of Lake Michigan, with emphasis on two exotic prey items

The trophic linkage between yellow perch Perca flavescens and two exotic prey items, alewife Alosa pseudoharengus and round goby Neogobius melanostomus , was investigated in the extreme southern area of Lake Michigan during the summer of 2002. Yellow perch ≥100 mm total length, L _T( n  = 1293) exhibited size selective feeding, with 148 fish containing round gobies and 120 fish containing alewives. The mean round goby L _T, preyed on by yellow perch, was 23% of the predator L _T, with a range of 7 to 47%, and mean alewife L _T was 32% of yellow perch L _T, with a range of 18 to 46%. Although the selection of prey size by yellow perch increased proportionally with yellow perch L _T, prey consumed appeared smaller than theoretically possible based on gape size.     

Distribution, growth, diet and foraging behaviour of the yellow-fin notothen Patagonotothen guntheri (Norman) on the Shag Rocks shelf (Southern Ocean)

The distribution, total length ( L _T) frequency and diet of Patagonotothen guntheri are described from 14 bottom trawl surveys conducted on the Shag Rocks and South Georgia shelves in the austral summers from 1986 to 2006. Patagonotothen guntheri (80–265 mm L _T) were caught on the Shag Rocks shelf from depths of 111 to 470 m, but no specimens were caught on the South Georgia shelf. Multiple cohorts were present during each survey and L _T-frequency analysis of these cohorts suggests that growth was slow (von Bertalanffy K = 0·133). Evidence from stomach contents and acoustic data (2005 and 2006) showed that P. guntheri is primarily a pelagic feeder, migrating from the sea floor towards the surface to feed during daylight. The diet of smaller fish (<140 mm) was dominated by copepods, predominantly Rhincalanus gigas , whilst larger fish principally consumed the pelagic hyperiid amphipod, Themisto gaudichaudii and Antarctic krill Euphausia superba . Some larger fish also took benthic prey.     

Comparison between micro- and macro-structure readings in the age estimations of Cape hake

By counting otolith daily increments juvenile Cape hake Merluccius capensis were found to grow faster than European hake Merluccius merluccius . The mean frequency of translucent zones in juveniles 130–240 mm L _T were higher (×3·2) than the annual frequency, although their mean duration was shorter (1 month) than a season. Comparison between micro- and macro-structure readings showed that the latter tended to overestimate the age of juvenile hake, with 1 year old hake being overestimated by 18% in individuals of 160 and 210 mm L _T.     

Reproductive cycle and sex inversion in razor fish, a protogynous labrid in the southern Mediterranean Sea

The reproductive biology of the Mediterranean razor fish Xyrichthys novacula was investigated by demographic data and histological analysis of the female, intersexual and male gonads. Specimens were collected by bottom trawl on a monthly basis between June 2000 and July 2001 in a sandy bay in southern Thyrrenian. Gonad histology confirmed that the Mediterranean razor fish is a monandric, protogynous hermaphrodite. Females reached first sexual maturity at 100 mm ( L _T) and the estimated mean L _T at first maturity ( L ₅₀) was 125 mm. Females exhibited asynchronous ovarian development and multiple ovulations occurred over the spawning period. Vitellogenesis started in early May and spawning occurred from late May until late September. Sexual transition involved a large‐scale atresia of all oocyte stages and a massive degeneration of ovarian tissue followed by primordial germ cells proliferation. Sex change began at spawning time (June) but transitional individuals tended to cluster at the end of the reproductive period (September). They accounted for 17·1% of the population sampled and were found in a broad size range (105–150 mm L _T).     

Variation in the diet of Genypterus blacodes(Ophidiidae) around the Falkland Islands

The diet change with size, season and area was investigated using the stomachs of 496 kingclip Genypterus blacodes collected around the Falkland Islands (south‐west Atlantic) between August 2001 and September 2002. The key prey species were rockcod Patagonotothen spp., benthic isopods and Patagonian grenadier Macruronus magellanicus . Kingclip <50 cm total length ( L _T) fed mainly on crustaceans and small fishes. With size the diet shifted away from crustaceans towards Patagonotothen spp. in kingclip 50–100 cm L _T, and finally towards larger fishes such as M. magellanicus and Micromesistius australis australis in kingclip >100 cm L _T. The niche breadth was highest in fish >100 cm L _T and the lowest in fish <50 cm L _T. The larger kingclip generally selected larger individuals of the same prey species, with the exception of the Patagonian squid Loligo gahi , where all ingested squid were of similar size, regardless of the predator length. The importance of the main prey species varied substantially between five consequent seasons studied, and appeared to follow the seasonal abundance and availability of prey. The spatial variability in the diet was found in kingclip caught in regions occupied by transformed temperate and sub‐Antarctic waters. The rockcod, which is available throughout the year around the Falkland Islands, was the most important prey in the kingclip diet. Kingclip takes advantage of other seasonally abundant prey species during their seasonal migrations ( e.g . L. gahi ) and also scavenge on discards from fishing vessels when available.     

Variation in food intake within groups of juvenile perch

The coefficient of variation of individual food intake was large in small groups of young-of-the-year perch Perca fluviatilis (45–60 mm L _T) and smaller when fish that were given the same feeding rank were placed together. No aggressive interactions were detected during feeding.     

Reproductive biology of the smooth-hound shark Mustelus mustelus (L.) in the Gulf of Gabès (south-central Mediterranean Sea)

The reproductive biology of the smooth-hound shark Mustelus mustelus was studied in the Gulf of Gabès (southern Tunisia). Females were found to mature between 1075 and 1230 mm total length ( L _T) whereas males matured between 880 and 1120 mm L _T. The L _T at which 50% of the population reached maturity was 971 and 1172 mm for males and females, respectively. Male gonads were symmetrical in terms of mass and both functional, whereas in females only the right ovary was functional. The seasonal changes in the oocytes and testes development, embryo length and the occurrence of near-term and post-partum females showed that this species displayed a clearly defined annual reproductive cycle with parturition occurring during late April and early May, after a gestation period of 10–11 months. Mating took place during May and early June and fertilization occurred from early June to early July. The embryo sex ratio was not significantly different from unity. Litter size varied from four to 18 embryos and was positively correlated with maternal L _T. The young were born with a L _T of 340–420 mm.     

A hydro-mechanical approach to the scaling of swimming performance in the sand flathead Platycephalus bassensis Cuvier: effects of changes in morphological features based on fish size

The swimming performance of Platycephalus bassensis at steady speed was assessed with an emphasis on hydrodynamics. The minimum swimming speed to maintain hydrostatic equilibrium for P. bassensis of 0·271 m total length ( L _T) was calculated to be 1·06 L _T s⁻¹. At this speed, the required lift to support the mass of the fish was equivalent to 6·6% of the fish mass; 82·7% of which was created by the body as a hydrofoil, and the rest of which was created by the pelvic fins as hydrofoils. The minimum swimming speed decreased with the L _T of the fish and ranged from 1·15 L _T s⁻¹ for a fish of 0·209 m to 0·89 L _T s⁻¹ for a fish of 0·407 m. The forward movement per tail-beat cycle ( i.e. stride length) was described with an equation including quantities of morphological and hydro-mechanical relevance. This equation explained that stride length was increased by the effect of turbulence characterized by the Reynolds number and demonstrated the morphological and hydro-mechanical functional design of the fish for maximizing thrust and minimizing drag. The larger span of the caudal fin and caudal tail-beat amplitude was associated with larger stride length, whereas greater frictional drag was associated with smaller stride length.