Life‐history traits and population decline of the Atlantic mackerel Scomber scombrusin the Adriatic Sea

This study investigated demographic structure and reproductive characteristics of the Atlantic mackerel Scomber scombrus, in relation to landing trends in the northern‐central Adriatic Sea. Results highlighted the occurrence of only small‐sized and young‐age individuals, and a marked decline from the 1990s to the present in maximum age (from 8 to 3 years) and total length (L_T; from 420 to 360 mm). Fecundity ranged between 40 000 and 190 000 eggs, and was related to female L_T. High levels of atresia implied lower values of actual fecundity. Sexual maturity was attained by 72·8% of individuals in their first year of life at 200 mm. The reduction in maximum L_T resulted in a marked decline in the population egg production, while the reduction in maximum age implied that females participated in fewer spawning events.     

The diet of Clinitrachus argentatus (Blennioidei: Clinidae) in the northern Aegean and Marmara Seas

Differences in the diet composition of small (28–42 mm total length, L_T) and large (43–75 mm L_T) individuals of the cline Clinitrachus argentatus (Clinidae) were evident. Smaller individuals preyed mainly on copepods, whereas larger individuals ate mainly amphipods, isopods, brachyurans and polychaetes.     

Living on the edge: latitudinal variations in the reproductive biology of two coastal species of sharks

Differences in the reproductive biology of both the Australian weasel shark Hemigaleus australiensis and the Australian sharpnose shark Rhizoprionodon taylori were apparent between individuals from the southern‐most extent of their range in eastern Australia (Moreton Bay) and those from northern Australia. For H. australiensis from Moreton Bay the total length (L_T) at which 50% of individuals were mature (L_T50) was 759 mm for females and 756 mm for males, values that were respectively 17–26% larger than reported for the species in northern Australia. The relatively low percentage (63%) of pregnant mature females and presence of small, similar‐sized, embryos in utero in both May and November suggested a semi‐synchronous, annual reproductive cycle in Moreton Bay, whereas a synchronous, biannual reproductive cycle occurred in northern Australia. It is likely that H. australiensis has a resting phase between gestation cycles at the southern‐most extent of its range. For R. taylori from Moreton Bay the L_T50s were 588 and 579 mm for females and males, respectively, values 2–3% larger than for individuals from the mid‐Queensland coast and 31–35% larger than for individuals from northern Australia. The length at which 50% of the females were maternal (611 mm L_T) in Moreton Bay was greater than the L_T50, indicating that not all sharks mate immediately after maturing. Rhizoprionodon taylori in the south had an annual reproductive cycle incorporating a 7–8 month embryonic diapause, with pups probably born in February. A mean fecundity of 7·5 was almost double that reported from northern Australia. Regional variations in the reproductive characteristics of H. australiensis and R. taylori may influence their resilience to fishing and other anthropogenic pressures. The substantial differences reported here highlight the importance of region‐specific life‐history parameters to successful management and conservation.     

Reproductive biology of Haemulon plumierii in the south‐western Atlantic Ocean's most extensive reefs: implications for fisheries management

The reproductive biology of the white grunt Haemulon plumierii was studied from 360 individuals obtained from artisanal fisheries landings in the Abrolhos Bank, Brazil, between August 2010 and March 2012. The overall sex‐ratio did not differ significantly from 1:1, although males predominated in larger size classes. β‐Binomial modelling of historical sex‐ratio data indicated that the catch rate of females has increased in recent years. Females reached maturity at a smaller total length (L_T; 214 mm) than males (235 mm L_T) and the L_T at which 50% of all individuals are mature (L₅₀) was 220 mm, corresponding to 41·5% of the maximum recorded L_T. Variation in the gonado‐somatic index and in the relative frequency of reproductive stages indicates that reproduction occurs year round, with increased activity during the austral spring and summer. Fecundity was not size dependent. The reproductive parameters provided here can support management measures focussed on seasonal closures during spawning peaks (September to November and February to March) and minimum sizes (>L₅₀) for the capture of this important artisanal fisheries resource in Abrolhos, the region with the largest and most biodiverse coralline reefs in the South Atlantic Ocean.     

Occurrence of age‐0 year dwarf pikeperch Sander lucioperca in late summer – an overlooked phenomenon in reservoirs

Late summer sampling of pelagic age‐0 year fish communities in five Czech reservoirs and one Dutch reservoir revealed extremely small age‐0 year pikeperch Sander lucioperca (mean 24 mm standard length, L_S, minimum 13 mm L_S) alongside more normal‐sized S. lucioperca that are found at the end of the first growing season (mean 50 mm L_S), resulting in two clearly size‐separated cohorts. Reference to such small age‐0 year S. lucioperca in lakes or reservoirs at this time of year and in such large numbers are almost absent the scientific literature, and the presence of these small S. lucioperca is contradictory to the common understanding of the reproductive biology of this species. This overlooked phenomenon may have a major effect on the population dynamics of this valuable species because of size‐dependent winter mortality.     

Dietary ontogeny and niche shift to piscivory in lacustrine brown trout Salmo trutta revealed by stomach content and stable isotope analyses

The feeding ecology and ontogeny of a large size range of brown trout Salmo trutta in Lake Fyresvatnet, southern Norway, were examined by stomach content and stable isotope analyses. According to the stomach contents, the S. trutta changed their diet at c. 30 cm total length (L_T). The smaller size classes fed on benthic invertebrates and surface insects, whereas larger S. trutta (>30 cm) fed mainly on whitefish Coregonus lavaretus. A similar, but more gradual shift to piscivory in the size range 25–30 cm was found when using the stable isotope mixing model SIAR to reveal dietary ontogeny. The δ¹³C isotopic signature confirmed that S. trutta independent of size predominantly relied upon benthic energy sources, suggesting that the littoral zone was the primary foraging habitat for both invertebrate and piscivorous feeders. The δ¹⁵N values and trophic position increased with predator length, ranging from an average of 3·60 for small‐sized S. trutta (<15 cm) to 4·15 for large‐sized fish (>35 cm). The S. trutta exhibited a relatively slow growth rate during the predominant invertebrate feeding stages up to 7 years of age and 28 cm L_T, whereas fish above this size and age displayed a rapid growth rate of 9–11 cm year^?1, demonstrating the profitability of piscivorous feeding.     

Population demographics of American eels Anguilla rostrata in two Arkansas,U.S.A., catchments that drain into the Gulf of Mexico

The goal of this study was to compare American eel Anguilla rostrata life history in two inland river systems in Arkansas, U.S.A., that ultimately discharge into the Gulf of Mexico via the Mississippi River and the Red‐Atchafalaya catchments. From 21 June 2011 to 24 April 2014, 238 yellow‐phase A. rostrata were captured in the middle Ouachita River and tributaries using boat electrofishing and 39 in the lower White River using multiple sampling gears. Most of them were caught downstream of dams in both basins (61%). Medium‐sized A. rostrata ranging from 225 to 350 mm total length (L_T) were the most abundant size group in the Ouachita River basin, but they were absent from the White River. Mean L_T at age 4 years (i.e. youngest shared age) was 150 mm greater for the White River than the Ouachita River basin. Anguilla rostrata appeared to have a greater initial L_T (i.e. minimum size upon arrival) in the White River that allowed them to reach a gonado‐somatic index (I_G) of 1·5 up to 4 years earlier, and downstream migration appeared to occur 5 years earlier at 100 mm greater L_T; these differences may be related to increased river fragmentation by dams in the Ouachita River basin. Growth and maturation of A. rostrata in this study were more similar to southern populations along the Atlantic coast than other inland populations. Adult swimbladder nematodes Anguillicoloides crassus were not present in any of the 214 swimbladders inspected. Gulf of Mexico catchments may be valuable production areas for A. rostrata and data from these systems should be considered as range‐wide protection and management plans are being developed.     

Larval and juvenile development of dusky grouper Epinephelus marginatus reared in mesocosms

The larval development of the dusky grouper Epinephelus marginatus up to the benthic juvenile stage is described in detail to establish a reference for their larval identification. Development is described in terms of ontogenetic changes in morphology, growth, pigmentation, fin structure and skeletal structure. Larvae were reared in mesocosms at a mean temperature of 24·3° C, salinity of 36·5, dissolved oxygen of 6·4 mg l^?1 and pH of 8·2. Newly hatched larvae had an estimated total length (L_T) of 2·3 mm. On the second day post hatching the yolk was almost fully absorbed with traces of the oil globule still present, the eyes were already pigmented and mouth and gut functional. At this stage the cranial skeletal elements for feeding and breathing (mouth and gills) and the pectoral‐fin support were already present. About 50% of the observed larvae had food in their guts. Pigmentation was very characteristic, consisting of two large chromatophores visible on the edge of the primordial fin, close to the midpoint of the post‐anal region of the body and over the midgut and hindgut and post‐anal portion of the body. At 2·9 mm L_T the emergence of the second dorsal‐fin spine, characteristic of the Epinephilinae, was clearly visible. The pre‐flexion stage started in larva of 3·2 mm L_T. At 5·5 mm L_T the larvae possessed posterior preopercular angle spines, and the dorsal and pelvic spines presented serrated edges and were pigmented. The water surface‐tension‐related death of the yolk sac and pre‐flexion larvae described in the rearing of several other grouper species did not occur during E. marginatus culture. Notochord flexion, with initial ossification of the caudal‐fin supporting elements, started at 6·6 mm L_T. At this stage the major melanophores, preopercular, dorsal and pelvic spines and mandibular teeth were already present. Transformation of larvae into juveniles occurred when larvae averaged 13·8 mm L_T. Juveniles with a mean L_T of 20·1 mm started to settle and most of them were benthic with a mean L_T of 26·8 mm.     

Variation in social information use: the influences of information reliability and mass on decision making in a group‐living fish Gambusia affinis

The effect of the reliability of available social information was assessed by examining whether the age of social information changes its effects on a foraging decision in a group‐living fish Gambusia affinis. Individuals switched their patch preference when faced with social information that conflicted with personal information in general; the age of the social information, however, did not significantly influence preference for feeding patch. The mass of decision makers was positively correlated with their use of available social information, with heavier individuals exhibiting a greater difference in patch preference than lighter individuals, suggesting that large and small G. affinis trade‐off the benefits of information acquisition and the costs of competition from conspecifics differently.     

Diel vertical migrations of bathypelagic perch fry

The behaviour of young‐of‐the‐year (YOY) perch Perca fluviatilis as a dominant species in the assemblage of fry in the pelagic of Slapy Reservoir (Czech Republic), was studied during late May and mid‐June 2002 using acoustic methods and complementary net catches. During the day, perch fry were present simultaneously in littoral, epipelagic and bathypelagic habitats. Bathypelagic perch fry, forming a scattering layer, migrated vertically each day between the epilimnion and hypolimnion, with an amplitude of 11·0 m in May and 12·5 m in June. At dusk, the migratory bathypelagic fry mixed in the epilimnion with non‐migrating epipelagic fry and spent the night close to the thermocline (abundance maximum at 3–4 m in May, 0–2 m in June). In June, shoaling behaviour by some of the bathypelagic perch fry was also observed: the shoaling fry remained higher in the water column than the non‐shoaling fry. Both depths of the scattering layer and the depths of the fry shoals were strongly controlled by the light intensity. The contribution of the bathypelagic part of the population to the total numbers of pelagic perch fry decreased from 28·1% in May to 4·7% in June, while the density of all pelagic perch fry increased (c. 96 000 individuals ha^?1 in May and 142 000 individuals ha^?1 in June). In May, the bathypelagic (average total length, L_T, 11·9 mm) and epipelagic (average L_T 14·6 mm) perch fry differed in size while, in June, the epipelagic fry were divided into two distinct size groups. The more abundant group, of small epipelagic perch fry (average L_T 14·6 mm), was similar in size to the bathypelagic fry (average L_T 14·6 mm) while the less abundant group, of larger epipelagic fry (average L_T 34·4 mm), was similar in size to littoral perch fry (average L_T 35·0 mm). The results suggest that in perch fry three different survival strategies with different risks can be used in the same locality, time and year.     

Size‐dependent reproductive success of wild zebrafish Danio rerio in the laboratory

Size‐dependent reproductive success of wild zebrafish Danio rerio was studied under controlled conditions in the laboratory to further understand the influence of spawner body size on reproductive output and egg and larval traits. Three different spawner size categories attained by size‐selective harvesting of the F₁‐offspring of wild D. rerio were established and their reproductive performance compared during a 5 day period. As to be expected, large females spawned more frequently and had significantly greater clutch sizes than small females. Contrary to expectations, small females produced larger eggs when measured as egg diameter with similar amounts of yolk compared to eggs spawned by large spawners. Eggs from small fish, however, suffered from higher egg mortality than the eggs of large individuals. Embryos from small‐sized spawners also hatched later than offspring from eggs laid by large females. Larval standard length (L_S)‐at‐hatch did not differ between the size categories, but the offspring of the large fish had significantly larger area‐at‐hatch and greater yolk‐sac volume indicating better condition. Offspring growth rates were generally similar between offspring from all size categories, but they were significantly higher for offspring spawned by small females in terms of L_S between days 60 and 90 post‐fertilization. Despite temporarily higher growth rates among the small fish offspring, the smaller energy reserves at hatching translated into lower condition later in ontogeny. It appeared that the influence of spawner body size on egg and larval traits was relatively pronounced early in development and seemed to remain in terms of condition, but not in growth, after the onset of exogenous feeding. Further studies are needed to explore the mechanisms behind the differences in offspring quality between large‐ and small‐sized spawners by disentangling size‐dependent maternal and paternal effects on reproductive variables in D. rerio.     

Catch composition and reproductive biology of Sphyrna lewini (Griffith & Smith) (Carcharhiniformes, Sphyrnidae) in Indonesian waters

Data on the length compositions and reproductive biology have been collected for Sphyrna lewini in Indonesian waters, in which this species makes an important contribution to the biomass of its artisanal and small‐scale fisheries. These data were obtained by recording relevant body measurements and counts for this species while visiting Indonesian fish landing sites. The fish, which had been caught by gillnetting and longlining, covered a wide length range and included substantial numbers of immature, maturing and mature individuals. The vast majority of S. lewini <1100 mm total length (L_T) had been caught by gillnetting, whereas most of those above this length had been obtained through longlining. The number of embryos in pregnant females, which ranged from 14 to 41, with a mean of 25, was positively correlated with the L_T of those females. Birth occurred at c. 400 mm L_T and predominantly in October and November. The number of females was similar to that of males among smaller fish, but far greater than that of males among larger fish. This presumably reflects a faster growth and greater longevity of females, selectivity of longlining for females and a tendency for males to move outside the area fished. For males, the relationships between clasper length and the extent of its calcification, and also with sexual maturation and L_T have been determined. Females attained maturity (L_T50) at a far larger size (2285 mm) than males (1756 mm). The L_T of virtually all females and all males taken by gillnetting were less than their respective L_T50 at maturity, and c. 67 and 51% of the longline catches of females and males, respectively, were likewise immature. This feature, together with the substantial catches of S. lewini and the life cycle traits of elasmobranchs, suggest that this species is likely to be prone to overfishing in Indonesian waters. Furthermore, the removal of large numbers of this apex predator will presumably be affecting the trophic structure in the waters in which it is fished.     

Reproductive biology of the southern thorny skate Amblyraja doellojuradoi (Chondrichthyes,Rajidae)

The total lengths (L_T) of 193 males (209–556 mm) and 130 females (275–515 mm) of Amblyraja doellojuradoi, a commercial by‐catch species on the Argentinean continental shelf, which are increasingly retained, were analysed. No sexual dimorphism was observed in the L_T at which 50% of individuals were sexually mature; males matured at 448 mm and females at 411 mm, c. 80 and 82% of maximum L_T. The hepato‐somatic index was similar among sexes, but significantly different between maturity stages, being lower in mature than immature specimens. Males had no seasonal difference in the hepato‐somatic index and females had the lowest index in autumn. The gonado‐somatic index was lower in males than in females and significantly higher in mature than immature specimens of both sexes. Males had the highest index in autumn and females had no seasonal difference. Collectively, these results would indicate that A. doellojuradoi breeds in autumn.     

Biological observations on the broadfin shark Lamiopsis temminckii (Carcharhiniformes: Carcharhinidae)

Biological information was collected from 214 individuals of the broadfin shark Lamiopsis temminckii measuring 418 to 1782 mm total length, L_T. Size at maturity (L₅₀) for females and males was estimated at 1430 and 1368 mm L_T, respectively, while mature and gravid females were observed from 1350 mm L_T with litter sizes 2–8 and size at birth 418–650 mm L_T. Analysis of stomach contents revealed a variety of prey, primarily crustaceans (54·0%), teleosts (42·7%) and cephalopods.     

Acoustic ontogeny of a teleost

The influence of growth on the intensity and variability of acoustic echoes from individuals and groups of a teleosts was quantified using Donaldson trout (rainbow–steelhead hybrid) Oncorhynchus mykiss. Fish growth was linear in total length (L_T) and quadratic in mass. Dorsal swimbladder area increased exponentially with L_T. Allometric growth ratio (i.e. k) values of swimbladder length linearly increased with L_T. Average swimbladder volumes occupied 3–6% of fish body volume and increased exponentially with L_T. The aspect angle that resulted in the maximum average acoustic intensity from the group shifted from 80 to 86° through the experimental period. Mean echo intensities increased at both 38 and 120 kHz as mean L_T increased. Predicted echo intensities at 38 kHz exceeded that at 120 kHz at L_T <150 mm but were less than that predicted at 120 kHz at L_T >280 mm. Generalized additive mixed models using L_T, swimbladder angles and lateral elongation ratios of fish bodies were better predictors of echo intensities than L_T alone.     

Biological observations on the bristly catshark Bythaelurus hispidus from deep waters off the south‐west coast of India

Biological data are presented for the poorly known bristly catshark Bythaelurus hispidus based on specimens collected from the by‐catch of the commercial deep‐sea shrimp trawl fishery operating in the Arabian Sea at depths of 200–500 m off the south‐west coast of India. One hundred and sixty‐two individuals, which ranged from 120 to 366 mm total length (L_T), were collected for this study. Size‐at‐maturity (L₅₀) for females and males was estimated at 252 and 235 mm L_T, respectively. The reproductive mode of B. hispidus was aplacental viviparity, which is the rarest reproductive mode within the Scyliorhinidae and is considered to be the most advanced of the three reproductive modes occurring within this family. Dietary analysis of stomach contents revealed B. hispidus feeds on a variety of prey, primarily fishes.     

A rare occurrence of reversal in the common megrim Lepidorhombus whiffiagonis (Pleuronectiformes: Scophthalmidae) in the northern North Sea

An adult (231 mm standard length, L_S; 371 mm total length, L_T), dextral (right‐sided) common megrim Lepidorhombus whiffiagonis is reported from the northern North Sea. This is the first recorded occurrence of reversal in this species. Other than its reversed asymmetry, the specimen had morphological and meristic characteristics similar to those of non‐reversed individuals of the same species.     

Evidence of facultative daytime hypothermia in a small passerine wintering at northern latitudes

The use of hypothermia as a means to save energy is well documented in birds. This energy‐saving strategy is widely considered to occur exclusively at night in diurnally active species. However, recent studies suggest that facultative hypothermia may also occur during the day. Here, we document the use of daytime hypothermia in foraging Black‐capped Chickadees Poecile atricapillus wintering in eastern Canada. We measured the body temperature (T_b) of 126 individuals (plus 48 repeated measures) during a single winter and related values to ambient temperature (T_a) at the time of capture. We also tested whether daytime hypothermia was correlated with the size of body reserves (residuals of mass on structural size and fat score) and levels of metabolic performance (basal metabolic rate and maximum thermogenic capacity). We found that T_b of individual birds was lower when captured at low T_a, reaching values as low as 35.5 °C in actively foraging individuals. T_b was unrelated to metabolic performance or measures of body reserves. Therefore, daytime hypothermia does not result from individuals being unable to maintain T_b during cold spells or to a lack of body reserves. Our data also demonstrated a high level of individual variation in the depth of hypothermia, the causes of which remain to be explored.     

Reproductive biology of the fat snook Centropomus parallelus Poey, 1860 (Teleostei,Centropomidae) and implications for its management in the southern Atlantic Ocean

The reproductive biology of Centropomus parallelus was described from 589 individuals captured in estuarine and coastal waters in Southern Brazil. Length-frequency distribution showed the dominance of males in smaller length-classes (132–290 mm L_T), whereas females were dominant in larger length-classes (>290 mm L_T). Total length at maturity (L₅₀) was 180 mm L_T and corresponded to 29% of the maximum length recorded. Histological sections revealed one hermaphrodite (205 mm L_T) and few immature females. Life history traits provided herein can contribute to sustainable fisheries management practices.     

Spatio‐temporal variability in the cannibalistic behaviour of European hake Merluccius merluccius: the influence of recruit abundance and prey availability

Cannibalistic behaviour of European hake Merluccius merluccius was studied through the analysis of 49 836 gut contents belonging to individuals from 6 to 82 cm in total length (L_T). Samples were collected every autumn between 1993 and 2009. The results showed that the consumption of conspecific individuals was consistent over space and time. The abundance, spatial distribution patterns and L_T structure of M. merluccius recruits were the main variables involved in M. merluccius cannibalism. A geographical pattern was found since increasing cannibalism was observed in areas of recruit aggregations. The L_T spectrum of recruits in autumn was also a key factor and dependent on the spawning period. When adults spawned from late spring to summer, an increasing cannibalism trend was found in autumn, due to the ideal size structure of the prey (M. merluccius recruits) for predators. Depth was also a significant variable, and a cannibal peak was detected at depths ranging between 50 and 200 m, coinciding with a spatial overlap of predator (pre‐adults) and prey (recruits). The cannibalistic behaviour of M. merluccius began at c. 12 cm, although 75% were mid‐sized individuals between 16 and 30 cm. Additionally, 90% of prey‐hake were recruits <18·5 cm. Some biological and ecological aspects such as growth rate, spawning period, predation and competition interactions and the effect of some oceanographic events on M. merluccius cannibalism are also discussed.