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 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.     

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).     

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.     

Early growth and juvenile population structure of lemon sharks Negaprion brevirostris in the Atol das Rocas Biological Reserve, off north‐east Brazil

Lemon sharks Negaprion brevirostris were sampled in the Atol das Rocas, a nursery area, on nine occasions from March 1999 to October 2003, during which 157 individuals were tagged and 35 were recaptured. The male : female sex ratio of captured individuals was 1 : 1·12. Mean ±  s . d . growth rates were 24·7 ± 3·4 cm year⁻¹ in total length ( L _T), 20·7 ± 3·2 cm year⁻¹ in fork length, and 19·5 ± 2·7 cm year⁻¹ in precaudal length. There was no significant difference in growth rates between males and females. Mean ±  s . d . increase in mass was 2565 ± 762 g year⁻¹. The von Bertalanffy growth parameters estimated by the Fabens method based on L _T were: k  = 0·077, L _∞ = 399·9 cm and t ₀ = −2·16. Despite the large variation of environmental conditions, particularly of tidal range and currents, and the lack of protective mangrove cover in the nursery area at Atol das Rocas, juvenile lemon sharks grew relatively faster than at other nurseries. Such rapid growth could be a response to abundant food availability or high risk of predation by adults that enter the nursery area.     

Age and growth of the dusky grouper Epinephelus marginatus (Lowe 1834) in an exploited population of the western Mediterranean Sea

The age and growth of the dusky grouper, Epinephelus marginatus , in the Balearic Islands (western Mediterranean) were studied by otolith analysis from a sample of 358 specimens ranging in total length ( L _T) from 6·6 to 105·6 cm. The specimens came from commercial artisanal and recreational spear fisheries between 1999 and 2003. Otoliths grew asymmetrically throughout the range of L _T studied, showing a clear pattern of alternating translucent and opaque bands. Marginal increment analysis of specimens up to 8 years-old indicated that a single opaque band was formed each year during spring and summer. Whole otoliths allowed ageing specimens up to 10 years old, but above that age whole otoliths yielded lower age estimates than sectioned otoliths. The maximum estimated age was 61 years, which significantly extends the estimated life span of the species from a maximum of 36 years in a previous study. The von Bertalanffy growth parameters were estimated as L _∞= 95·5 cm L _T, K = 0·087 and T _O=−1·12. The study revealed differences in mean L _T at age and age structure between the shallow- and deep-water samples which may be attributed to different fishing pressure and environmental conditions.     

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.     

Population variables and life-history characteristics of the alligator pipefish Syngnathoides biaculeatus, in Papua New Guinea

Population structure and life-history variables of the widely distributed alligator pipefish Syngnathoides biaculeatus were characterized in Bootless Bay, Papua New Guinea over the course of 11 months. There was little evidence of seasonality with four focal populations showing no significant change in abundance. Similarly, the sex ratio remained 1:1 for all but 1 month. Reproductive males carrying eggs (148–278 mm in total length, L _T) were found in all months. Brood size was significantly, positively related to male L _T for newly laid broods only. Maximum observed brood size was 351 and mean ± s . d . brood size was 238 ± 57 for newly laid broods. Juveniles and males showed no change in mean L _T over the year while slightly smaller females were captured in November 2006 and September 2007. Males were significantly longer than females so von Bertalanffy growth coefficients were estimated separately for each sex: males L _∞= 285 mm, K = 0·82 year⁻¹ and females L _∞= 261 mm, K = 1·10 year⁻¹. These estimates suggest that this species grows rapidly and has a short-life span. In the context of growing concern about overexploitation of syngnathids, a rapid growth rate combined with year round reproductive activity suggests that the tropical S. biaculeatus may be relatively resilient with regard to fishing pressure.     

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.     

Sexual and geographical variation in life history parameters of the shorthorn sculpin

A total of 293 shorthorn sculpins Myoxocephalus scorpius from Tromsø, northern Norway, were sampled between November 1998 and April 1999 to determine sex, total length, age, growth, maturity and mortality. Females grew to larger sizes ( L _∞=26·9 v. 18·5 cm), matured later (2 v. 1 year of age) at larger size (maturation length=16 v. 14 cm L _T), and had lower instantaneous mortality rates (0·93 v. 1·20 year⁻¹) than males. The life history parameters of shorthorn sculpins in northern Norway were more similar to the parameters of short-lived central European populations than to the parameters of the long-lived population of Newfoundland. This study confirms that northern Norwegian shorthorn sculpins exhibit sexual dimorphism as in other shorthorn sculpin populations. The relationships between growth pattern, age at maturity and mortality rates observed in the Tromsø population and in other shorthorn sculpin populations, correspond well with the predictions from a published life history model.     

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.     

On the life history of the lesser gurnard (Scorpaeniformes: Triglidae) inhabiting the Agulhas Bank, South Africa

Growth analysis based on sectioned sagittal otoliths revealed the lesser gurnard Chelidonichthys queketti on the Agulhas Bank to be relatively fast growing and long lived, with ages of up to 7 years being recorded. Total length at age (mm) was described best by the specialized von Bertalanffy growth model as L _T=306·1 (1 − e^{0·53(t+0·18)}). First approximations of total, natural and fishing mortality rates were determined at 0·73, 0·38 and 0·35 year⁻¹ respectively. The adult population was male dominated with a sex ratio of 1 female: 1·2 males with the mean size of males and females being similar. The lesser gurnard is an iteroparous species with females maturing by the end of the first year of life (195 mm L _T), thereafter spawning throughout the year with reproductive activity peaking over spring and late summer. The lesser gurnard appears to exhibit similar life-history patterns to other triglid species in that it can be classified as a generalist. Generalistic life-history characteristics such as a fast growth rate, early sexual maturity at a relatively large size, a non-seasonal spawning pattern, feeding on a variety of prey organisms and the ability to inhabit various substrata could all contribute to it maintaining a high biomass on the Agulhas Bank.     

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.     

Effects of temperature, body size and feeding on rates of metabolism in young‐of‐the‐year haddock

The mean rate of oxygen consumption (routine respiration rate, R _R, mg O₂ fish⁻¹ h⁻¹), measured for individual or small groups of haddock Melanogrammus aeglefinus (3–12 cm standard length, L _S) maintained for 5 days within flow‐through respiratory chambers at four different temperatures, increased with increasing dry mass ( M _D). The relationship between R _R and M _D was allometric ( R _R = α  M ^b ) with b values of 0·631, 0·606, 0·655 and 0·650 at 5·0, 8·0, 12·0 and 15·0° C, respectively. The effect of temperature ( T ) and M _D on mean R _R was described by     indicating a Q ₁₀ of 2·27 between 5 and 15° C. Juvenile haddock routine metabolic scope, calculated as the ratio of the mean of highest and lowest deciles of R _R measured in each chamber, significantly decreased with temperature such that the routine scope at 15° C was half that at 5° C. The cost of feeding ( R _SDA) was c . 3% of consumed food energy, a value half that found for larger gadoid juveniles and adults.     

Growth of young-of-the-year mackerel in the Bay of Biscay

The first growth season of young-of-the-year (0+ year) mackerel Scomber scombrus , sampled in the Bay of Biscay, was parameterized to determine growth patterns. Daily increments were identified on sagittae otoliths, for calculation of age and growth of 92 larvae and 54 juveniles over the range 3·6–215·0 mm standard length ( L _S). A Gompertz curve was fitted to the length-at-age data. At the end of the first year of growth L _S was 194·2 mm, with a maximum growth increment of c . 2 mm day⁻¹, observed 62 days after hatching. Backcalculated growth increments for mackerel juveniles, during their larval stage, were higher than those observed for sampled larvae; only 10·9% of sampled larvae were estimated to survive. Growth for north-eastern Atlantic mackerel was slower than that published for north-western Atlantic mackerel. Backcalculated hatching dates for mackerel were consistent with the typical temporal distribution of mackerel spawning in the Bay of Biscay.     

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.     

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.     

Growth response of pikeperch larvae in relation to body size and zooplankton abundance

The most critical period at onset of feeding in pikeperch Stizostedion lucioperca is short (<5 days at 20° C). The larvae are sensitive to prey density during the first week of exogenous feeding. First-feeding larvae of 6.5 mm total length ( L_T ) needed prey densities of >585 prey l⁻¹ to maintain mass (C_maint), whereas 5 days older larvae of 7 mm L _T C_maint= 55 prey l⁻¹ and for 11 mm L _T larvae C_maint<10 prey 1⁻¹. Changes in specific growth rates for larvae of 7 and 11 mm were similar to a type-II functional response curve reaching a specific growth rate of 26 and 30% day⁻¹, respectively, at 1000 prey l⁻¹, whereas the 6·5 mm larvae showed a linear growth response reaching a specific growth rate of only 9% day ⁻¹ at 1000 prey l⁻¹. The results suggest that prey density is a limiting factor, which might contribute to the high variation between year-class Strengths.     

The reproduction, age and growth of the spotted rockling

Biology of the Macaronesian endemic rockling Gaidropsarus guttatus was studied in the Azores. The overall sex ratio from the samples was highly in favour of females (1 : 6·33). The growth parameters were L _∞ = 24·23, k  = 1·219 and t ₀ = −0·059. Fish matured at 15 cm L _T and the spawning season was strongly clustered in April.