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).     

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.     

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.     

Diet and prey size spectrum of pikeperch in lakes in central Finland

The stomach content of pikeperch Sander lucioperca was studied in 24 lakes in central Finland. The most important prey species was smelt Osmerus eperlanus then perch Perca fluviatilis . The proportion of cyprinids as prey increased with pikeperch total length ( L _T) and lake phosphorus content. Prey L _T and pikeperch L _T correlated positively, but the prey‐to‐predator size ratio correlated negatively with pikeperch L _T.     

Reproductive biology of the milk shark Rhizoprionodon acutus and the bigeye houndshark Iago omanensis in the coastal waters of Oman

Female Rhizoprionodon acutus were found to mature between 62 and 74 cm total length ( L _T) whereas males matured between 63 and 71 cm L _T. The L _T at 50% maturity was 64·3 cm for females and 64·7 cm for males. Litter size varied from one to six embryos, and was positively correlated with maternal L _T. Female embryos outnumbered males by a ratio of 2·3:1. The size at birth was c . 37 cm L _T. Full‐term embryos and post‐partum females were observed during all seasons although their occurrence was highest in spring. Spermatozoa were rarely recorded in the oviducal gland, indicating that this species does not store sperm. It was not possible to generate maturity curves for Iago omanensis but it was evident that females matured by the time they reached 35 cm and males were mature by 31 cm L _T. This species displayed a clearly defined reproductive cycle with parturition occurring primarily in spring, after a gestation period of c . 1 year. Maximal embryo size was 19 cm L _T while maximal litter size was 24 embryos. The oviducal gland appeared to act as a seminal receptacle and it appeared that females may utilize these stores by not mating every year.     

Diet of brown trout in relation to variation in abundance and size of pelagic fish prey

The seasonal diet of a predator, brown trout Salmo trutta [total length ( L _T) 17–69 cm] and simultaneous density and size‐structure of prey populations, vendace Coregonus albula and smelt Osmerus eperlanus (4–16 cm L _T), in a large boreal lake were analysed and compared in 2001 and 2002. The upper L _T limit for consumed prey was c . 40% of the predator L _T. All brown trout, however, preferred small (<10 cm L _T) and avoided large (≥10 cm L _T) prey. The results also suggested that equal densities of similar‐sized (4–10 cm L _T) fish of the two prey species led to random foraging on these species by brown trout, but if either one of the prey species predominated (>50%) in the lake, brown trout shifted to foraging on this species almost exclusively. Brown trout diets thus reflected the density dynamics of the two alternative prey species.     

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.     

A histological study on the development of the digestive system of Pseudosciaena crocea larvae and juveniles

The ontogenetic development of the gut and accessory organs in large yellow croaker Pseudosciaena crocea was investigated using light microscopy from hatching up to the juvenile stage (40 days post hatch, dph). At 3 dph (mean ±  s . d ., 4·1 ± 0·1 mm total length, L _T), coinciding with the buccopharynx opening, larvae started to feed exogenously, and the gut consisted of a well‐developed buccopharynx, a partially‐differentiated oesophagus and an intestine divided in three regions (anterior intestine, intermediate intestine and rectum). Yolk reserves were not completely depleted at the onset of exogenous feeding, and a period of mixed nutrition was observed up to 6 dph (4·3 ± 0·1 mm L _T), when yolk was definitively exhausted. Important morphological changes occurred at the end of the larval period, coinciding with metamorphosis. At 17 dph (6·8 ± 0·6 mm L _T), pyloric caeca differentiated at the junction of the pyloric stomach and the anterior intestine. Gastric glands were first observed at 21 dph (9·2 ± 1·2 mm L _T), coinciding with the morphological development of the stomach in three different regions (cardiac, fundic and pyloric) according to the histological characteristics of their mucosa. At this age, large longitudinal folds appeared in the median and posterior oesophageal mucosa. These morphological and histological features suggested the achievement of a digestive system characteristic of large yellow croaker juveniles and adults.     

Mackerel icefish size and age differences and long‐term change at South Georgia and Shag Rocks

This study analysed the total length ( L _T)‐frequency distribution of mackerel icefish Champsocephalus gunnari at South Georgia and Shag Rocks from nine bottom trawl surveys at South Georgia and eight at Shag Rocks between 1987 and 2002. The estimated mean L _T of age‐classes 1+, 2+, 3+ and 4+ years during January were, respectively, 14·7, 23·5, 29·8 and 35·1 cm at South Georgia. Age‐classes 1+, 2+ and 3+ years were 18·3, 26·2 and 33·8 cm at Shag Rocks. The derived Bertalanffy growth parameters for South Georgia were: L _∞ = 51·7 cm, k  = 0·27 and t ₀ = −0·26. The mean L _T of each age‐class of C. gunnari at Shag Rocks was significantly larger than at South Georgia, equivalent to c . 5 months growth, although the annual growth in L _T was similar. This is further evidence that C. gunnari hatched earlier at Shag Rocks. At South Georgia, the mean L _T of age‐classes 1+ and 3+ years were correlated, and significantly decreased between 1987 and 2002, and were smaller following warmer summers. This decrease in the size of C. gunnari may be the result of reduced food availability linked to climate warming.     

Feeding ecology of silver hake larvae on the Western Bank, Scotian Shelf, and comparison with Atlantic Cod

The feeding ecology of the larvae of silver hake Merluccius bilinearis was examined during two time periods (October 1998 and December 1992) on the Western Bank, Scotian Shelf, north‐west Atlantic, and compared with the feeding ecology of Atlantic cod Gadus morhua larvae collected in the same samples in December 1992. During both time periods silver hake exhibited strong selection for late stage copepodids and adult copepods at a small size (>3·5 mm total length, L _T). The niche width measured as the diet breadth index ( I _DB) of silver hake declined rapidly as they increased in size and remained relatively constant from 3 to 11 mm L _T, during each time period. Atlantic cod larvae exhibited a broader niche width that was curvilinear over the same L _T. Atlantic cod were also less selective than silver hake, incorporating both naupliar and early stage copepodids in their diets throughout the length classes examined. Simple isometric relationships did not explain the differences in diet, as Atlantic cod larvae continued to feed on early stages of copepods at large size, while silver hake larvae quickly switched to large prey items. The strong selection and narrow I _DB observed for silver hake probably reflects adaptation to spawning during the periods between major secondary production peaks in temperate waters.     

Age and growth of Anguilla anguilla in the Camargue lagoons

Age and total length ( L _T) data from a 11 year monitoring of the Anguilla anguilla eel population of the Camargue lagoons (Rhône delta, southern France) were collected for glass, yellow and silver eels. Three distinct models were calibrated to describe the growth process of undifferentiated eels, females and males, respectively. Uncertainty of parameter estimates was evaluated by bootstrapping. Females were characterized by larger asymptotic body size ( L _T) than males (580 ± 50 v . 388 ± 13 mm) and faster growth, whilst the Brody growth coefficient was larger for males than for females (means ±  s . d . 3·00 10⁻³ ± 1·68 10⁻³ v . 1·73 10⁻³ ± 0·50 10⁻³). Sexual differentiation was estimated to begin at 204 ± 38 mm mean ±  s . d ., i.e . at the end of the second year in the lagoons, well before the L _T at which macroscopic differentiation became possible ( c . 300 mm). Males probably leave the lagoon or die (due to either natural or fishing mortality) within the first 3 years, whilst females can remain up to 5 years. Sexual differentiation and maturation have a major role in shaping the L _T structure of the population. The L _T and mass ( M ) data were fitted by allometric curves     . The calibration of distinct curves for data from different years indicated that the allometric coefficient a was subject to wider interannual fluctuations than the allometric exponent b . A negative correlation linked the average L _T and the allometric exponent ( r  = −0·58, P  < 0·01).     

Life‐history variation among local populations of Atlantic cod from the Norwegian Skagerrak coast

Small‐scale spatial variation in life history was found among genetically distinct local populations of Atlantic cod Gadus morhua from the Norwegian Skagerrak coast. Among populations, age at 50% maturity varied from 2·6 to 3·8 years, total ( L _T) length at 50% maturity from 35 to 60 cm, annual survival from 33 to 64%, mean L _T at age 4 years from 43 to 63 cm, and mean backcalculated L _T at age 1 year from 8 to 12 cm.     

Reproductive biology and population variables of the Brazilian sharpnose shark Rhizoprionodon lalandii (Müller & Henle, 1839) captured in coastal waters of south-eastern Brazil

Throughout 1 year, from October 2003 to September 2004, 88 visits to the landing site of a small urban fishery (APREBAN) in Rio de Janeiro city were conducted and 816 specimens of Rhizoprionodon lalandii were analysed. The sample, mostly females, was composed of two cohorts: young-of-the-year were abundant in spring and summer and adults predominated in autumn and winter. Gravid females were most abundant from April to June, whereas post-partum females composed most of the catch in August to September. Adult males were present all year although were more abundant between February and July. No neonates were captured during the study and most embryos collected measured slightly below the reported total length ( L _T) at time of birth ( L ₀) for the species, suggesting that parturition may occur slightly outside the main fishing grounds or that neonates were not captured in commercial gillnets set at this time of the year. The mean L _T at maturity ( L _T50) for males was 578 mm and females matured between 620 and 660 mm, although a precise estimate of L _T50 for females could not be determined. The total length ( L _T) and total mass ( M _T) relationship     was calculated for both sexes and showed no significant differences. The mean condition factor increased steadily from February to July followed by a steep decrease in values relative to females in August and September, suggesting a pupping season. The present study area can be classified as a coastal juvenile habitat and a probable mating ground for R. lalandii .     

Changes in growth performance and proximate composition in Japanese flounder during metamorphosis

Morphological changes are described in Japanese flounder Paralichthys olivaceus larvae and juveniles with emphasis on growth during the period of metamorphosis. Ontogenetic changes in condition factor and lipid, glycogen and protein levels were also analysed to determine the utilization of stored energy. Fish grew from 6·6 to 20·3 mm L _T(1·15 to 84·4 mg in mass, M ) during the period from 11 to 40 days after hatching (DAH) at 19·3° C. Per cent specific growth rate per day (% G day⁻¹) for wet mass was lowest during the metamorphic phase (21–30 DAH) compared to pre‐(11–20 DAH) and post‐metamorphic (31–40 DAH) phases. When L _T and M were expressed as     , the b value was highest during the pre‐metamorphic phase and lowest during the metamorphic phase. These findings indicate that the developmental changes that occur during metamorphosis of Japanese flounder are closely related to the growth pattern. Moisture, lipid and glycogen contents were also at the lowest level during metamorphosis compared to pre‐ and post‐metamorphosis, which suggest that Japanese flounder use their energy reserves to accomplish metamorphosis due to an apparent decline in feeding during this period.     

Effect of temperature on swimming performance of sea bass juveniles

At four temperatures ( T= 15, 20, 25 and 28° C) swimming performance of Dicentrarchus labrax was significantly correlated with total length (23–43 mm L _T); r²=0.623–0.829). The relative critical swimming speed ( RU _crit= U _crit L _T⁻¹), where U _crit is the critical swimming speed, was constant throughout the L _T range studied. The significant effect of temperature on the relative critical swimming speed was described binomially: RU _crit=−0.0323T²+ 1.578 T −10.588 (r²=1). The estimated maximum RU _crit (8.69 L _T s⁻¹) was achieved at 24.4° C, and the 90% performance level was estimated between 19.3 and 29.6° C.     

Size-specific growth of bluegill, largemouth bass and channel catfish in relation to prey availability and limnological variables

Growth of sympatric populations of three important sport fish species: bluegill Lepomis macrochirus , largemouth bass Micropterus salmoides and channel catfish Ictalurus punctatus , in 14 Illinois reservoirs was assessed in an attempt to relate size-specific growth to environmental conditions. Multiple regression relationships for most species and size classes explained a large percentage of the variation in growth. Growth of small bluegill (50 mm total length, L _T) showed a strong negative relationship with bluegill catch per unit effort (cpue), per cent littoral area and pH. Large bluegill (150 mm L _T) growth was negatively related to Daphnia spp. and benthic macroinvertebrate abundance and lake volume, and positively related to bluegill cpue. Growth of small (100 mm L _T) and large (250 mm L _T) largemouth bass was not well explained by any of the measured variables. Growth of both small (300 mm L _T) and large (450 mm L _T) channel catfish was strongly positively related to forage fishes and ichthyoplankton abundance, and per cent littoral area while negatively related to benthic macroinvertebrates. By identifying environmental conditions associated with increased growth rates, these models provide direction for managing fish populations and suggest testable hypotheses for future study of the complex interactions between environmental conditions and growth.     

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.     

Effect of delayed first feeding on development and feeding ability of Paralabrax maculatofasciatus larvae

The impact of delayed feeding early in development on late feeding ability and development of spotted sand bass Paralabrax maculatofasciatus larvae was examined. Larvae were sampled from hatching until day 19 after delayed feeding for zero (control), 1, 2, or 3 days. Feeding incidence was evaluated as the percentage of larvae with food in the gut and feeding intensity was measured by direct counting of prey after dissection of the gut. Delayed feeding effects due to starvation were observed early in development. By day 5, significant differences ( P  < 0·05) were observed between controls and larvae submitted to degrees of delayed feeding, including total length ( L _T), eye diameter, and the size of the head, liver, gut, muscles and body. Differences were still apparent in L _T, and body, liver and muscle heights at the end of the studied period. Larvae under total starvation did not survive beyond day 5. Initial feeding incidence was 35, 60, 90 and 10% for larvae first fed on day 2, 3, 4 or 5, respectively. Mean ±  s . d . feeding intensity was 3·6 ± 0·8, 2·8 ± 1·3, 5·2 ± 0·3, and 10·2 ± 1·5 rotifers per larva depending on whether larvae were initially fed on day 2, 3, 4 or 5, respectively. The point of no return occurred between 4 and 5 days after hatching.     

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.     

Effects of DL–thyroxine on swimming speed in pearl danio Brachydanio albolineatus (Blyth)

Pearl Danios (Brachydanio albolineatus –Blyth) were allowed four days for acclimation in river-water filled aquaria maintained at a mean temperature of 27° C. A different concentration of DL-thyroxine (T₄) was added to each of two aquaria. A third aquarium served as a control and received no hormone. Photoperiod, feeding, aeration, and observer interference were controlled so as not to influence the results of the study. Mean swimming speeds for the fish in each aquarium were calculated. Data were subjected to analysis of variance and the Student-Newman-Keuls Test. Compared with the control, significant (P < 0.05) increases in swimming speed were found with concentrations of thyroxine in water of 1:4x 10⁶ and 1:1 x 10⁶. The difference in swimming speed between the two hormone-treated groups was not significant. The fact that exogenous thyroxine will increase swimming speed may indicate that endogenous thyroid hormone plays a significant role in such fish behaviour as migration.