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.     

Maturation of walleye by age, size and surplus energy

The probability of annual sexual maturation by male and female walleye Stizostedion vitreum was related to age, size and an index of condition, I _VF=[arcsine(visceral fat)^0·5(body mass)^−0·5]. Most males first matured at ages 2 and 3 years; size explained first maturation, but condition explained later maturation. In contrast, most females first matured at ages 4 and 5 years; maturity of females was more dependent upon condition. Maturity of females at ages 4 and 5 years was significantly correlated with average I _VF of the population ( PI _VF). The size reached by age 2 years (early growth) was correlated with the PI _VF. Growing degree-days, Secchi depth, latitude and lake morphology were not correlated with the PI _VF. Annual variations in female spawning stock size were related to the condition of the females, presumably reflecting the net acquisition of energy in the preceding growing season. Annual variations within lakes in the net acquisition of energy may exceed the variations in energy availability between lakes, dictated by lake morphology and geography. Thus, assessment of condition could be used to predict annual potential spawning stock size and egg production.     

Sea growth, smolt age and age at sexual maturation in Atlantic salmon

Relationships between growth at sea, smolt size and age at sexual maturation of Atlantic salmon Salmo salar were tested. The fish were offspring of brood stocks sampled in eight Norwegian rivers at latitudes between 59° and 70° N, hatchery reared and released at smolting at the mouth of the River Imsa (59° N). Smolt size influenced the subsequent growth rate of Atlantic salmon. The larger the fish were at release, the slower the yearly length increment at sea. Mean sea age at sexual maturity, measured as proportion of the returning adults attaining sexual maturity at sea age 2 years, was significantly correlated with mean growth rate during the first year at sea and mean smolt size ( r ²= 0·74, P < 0·001). Fish attaining maturity at a relatively high sea age were more fast growing during their first year at sea than those maturing at a younger age. The results indicate that high sea age at sexual maturation is a population-specific characteristic and associated with high early growth rate at sea.     

Reproductive characteristics and population decline of four species of skate (Rajidae) off the eastern coast of Canada

Four of the most common species of skate (Rajidae) were studied off eastern Canada to determine if their reproductive characteristics were linked to their population trajectories. The fecundity of the winter skate Leucoraja ocellata , the little skate Leucoraja erinacea , the thorny skate Amblyraja radiata and the smooth skate Malacoraja senta averaged between 41 and 56 egg cases per year for each species. For all species but L. ocellata , males matured at larger sizes and at later ages than females. Theoretical rates of population increase for non-equilibrium populations of L. ocellata ( c . 0·07), M. senta ( c . 0·14) and L. erinacea and A. radiata ( c . 0·20) were low compared to most fishes, indicating that north-west Atlantic skates are intrinsically unproductive, yet are theoretically capable of supporting low-level fisheries. Nevertheless, the results of 36 years of research surveys indicate that the abundance of mature L. ocellata , A. radiata and M. senta all decreased by >90% since 1970, indicating that past fishing mortality (both directed and undirected) has outstripped the net productivity of the skate populations on the eastern Scotian Shelf. The relationship between maximum age ( t _max) and age of maturity ( t _mat) was a better predictor of population growth rate than was body size, with the species exhibiting the highest ratios of t _mat : t _max ( L. ocellata = 0·68, M. senta = 0·66) having the lowest predicted population growth rates. L. ocellata appears to have the lowest productivity and has experienced the greatest population decline, thus raising concerns over its future status.     

Abundance and distribution of Diphyllobothrium ditremum Creplin (Cestoda: Pseudophyllidea) plerocercoids in benthic whitefish, in northern Finnish Lapland

The prevalence of Diphyllobothrium ditremum plerocercoids in whitefish Coregonus lavaretus ranged between 70 and 100% in Lake Kilpisjärvi and in three other lakes in Northern Lapland, Finland. The mean abundance in Lake Kilpisjärvi (age groups 1+-10+ years), ranged between 103·5± 71·3 in 1992–1993 to 110·9± 80·0 plerocercoids per fish in 1997. The asymptotic value of the infection levelled at 113 plerocercoids per host after age 3. No significant difference in abundance was detected between study years ( P >0·10). Abundances in other lakes ranged between 4·8±9·7 and 91·1±115·1. Two seasonal peaks of plerocercoid recruitment were observed in Lake Kilpisjärvi; between March and April ( P <0·002) and between September and October ( P =0·042). In autumn the numbers of larvae increased particularly in female fish. The invasion rate of the parasite was lower in other lakes studied, and the infection rate in whitefish was closely related to the copepod food eaten.     

Early survival and growth in populations of grayling with recent common ancestors—field experiments

Among eight populations of grayling Thymallus thymallus that shared common ancestors 8–28 generations earlier, mean egg mortality ranged between 1·2 and 59·8%; mortality during yolk sac absorption was low (0middot;0–8middot;0%) and mean survival to swim-up was high (90–97%), with two exceptions (20% and 50%). Survival probabilities differed significantly among populations, even after statistically adjusting for size at swim-up and water depth, water velocity, gravel size and temperature. Development time from fertilization to first observed swim-up was significantly different among populations, ranging between 27 to 40 days, or 264 to 280 degree-days. Mean specific growth rates during the first external feeding period varied between 1·2 and 2·0% day^-1 among populations. Population-specific growth rates were independent of temperature. Rates of divergence were of the same order as in other salmonid studies showing rapid evolution. The phenotypic differences may be due partly to genetic differentiation among the populations.     

Variations in age and size at maturity of female Nile tilapia, Oreochromis niloticus, populations from man-made lakes of Côte d'Ivoire

The reproductive characteristics of Oreochromis niloticus females were studied in two large hydroelectric dams and 6 small agropastoral reservoirs of Côte d'Ivoire, selected for their diversity of environmental conditions and sizes (6 to 80000 ha). Comparative analysis of age and size at maturity revealed large differences between the populations with early maturity, late maturity, and intermediate situations. All the populations matured in their first year, between 5.6 and 10 months. Age and size at maturity were positively correlated with the reservoir area. The range of variation in age at maturity between populations was far greater than that of size at maturity. However, size at maturity also appears to be very plastic as intra-population variations of 2 to 3 cm were found between consecutive years. A comparison of age at maturity for a population in 1994 and 1996 showed that this trait is also likely to vary significantly at the intra-population level. Environmental factors which could potentially affect age and size at maturity are discussed. Growth differences are the probable explanation for the observed patterns of variation between populations. Rapid changes in age and size at maturity at inter or intra-population levels suggest that the observed variations might be explained by differences in environmental variables (phenotypic plasticity) rather than by genetic differences.     

Egg cannibalism in Barents Sea capelin in relation to a narrow spawning distribution

The spawning distribution of Barents Sea capelin Mallotus villosus off northern Norway in 2002 was restricted to a narrow area at the easternmost spawning grounds. There was an increase in abundance and a marked shift in aggregation area from east towards west during 11 March to 4 April, as the capelin went from prespawning through spawning to spent stages. The capelin stomachs were either empty or contained capelin eggs, often mixed with sand. Both the occurrence of cannibals and stomach fullness increased with stage of maturity, being highest in spent fish, and higher in males than in females. No differences were found between cannibals and non-cannibals of the same sex when adjusted for total length ( L _T), age, L _T at age and condition factor. At similar stomach fullness, the females had consumed more eggs than the males. This was attributed to a higher proportion of empty and broken eggs and less sand in females compared to males. In the full stomachs, the mean consumption in females and males was 623 and 334 eggs, respectively, whereas the respective maximum egg consumption was 871 and 683 eggs. The minimum estimates (given no digestion) of mean ± 95% CL egg consumption in spent females and males were 75·4 ± 6·9 and 58·4 ± 12·0 eggs, respectively, implying a minimum mortality of 1–2% of the total egg production caused by cannibalism.     

Studies on the biology of reproduction in the cichlid Tilapia nilotica (L.): gonadal maturation and fecundity

Tilapia nilotica reaches maturity in the first year of life; females mature shortly before males (11·4 and 14·3 cm in length). The distinctive morphological features of the different stages of development of the gonads are characterized. The sex ratio varied in populations of different age (size)-groups: a ratio of 2·81:1 and 0·47:1 (females:males) was obtained in populations of young and old fish. In medium-sized populations the sexes were equally abundant. There is evidence that females breed more than once a season. Fecundity varied more with body length ( r =0·860) and weight ( r =0·806) than with age ( r =0·604). These variations were of the order of 2·02; 0·83 and 0·83 exponentials of length, weight and age, respectively.     

Life history constraints explain negative relationship between fish productivity and dissolved organic carbon in lakes

Resource availability constrains the life history strategies available to organisms and may thereby limit population growth rates and productivity. We used this conceptual framework to explore the mechanisms driving recently reported negative relationships between fish productivity and dissolved organic carbon (DOC) concentrations in lakes. We studied populations of bluegill (Lepomis macrochirus) in a set of lakes with DOC concentrations ranging from 3 to 24 mg/L; previous work has demonstrated that primary and secondary productivity of food webs is negatively related to DOC concentration across this gradient. For each population, we quantified individual growth rate, age at maturity, age‐specific fecundity, maximum age, length‐weight and length‐egg size relationships, and other life history characteristics. We observed a strong negative relationship between maximum size and DOC concentration; for instance, fish reached masses of 150 to 260 g in low‐DOC lakes but <120 g in high‐DOC lakes. Relationships between fecundity and length, and between egg size and length, were constant across the DOC gradient. Because fish in high‐DOC lakes reached smaller sizes but had similar fecundity and egg size at a given size, their total lifetime fecundity was as much as two orders of magnitude lower than fish in low‐DOC lakes. High DOC concentrations appeared to constrain the range of bluegill life history strategies available; populations in high‐DOC lakes always had low initial growth rates and high ages at maturity, whereas populations in low‐DOC showed higher variability in these traits. This was also the case for the intrinsic rates of natural increase of these populations, which were always low at the high end of the DOC gradient. The potentially lower capacity for fish populations in high‐DOC lakes to recover from exploitation has clear implications for the sustainable management of recreational fisheries in the face of considerable spatial heterogeneity and ongoing temporal change in lake DOC concentrations.     

Habitat use and life history of brown trout (Salmo trutta) and Arctic charr (Salvelinus alpinus) in some low acidity lakes in central Norway

Habitat utilization and the life history of browntrout Salmo trutta and Arctic charr Salvelinus alpinus were investigated in fivesympatric populations and five allopatric brown troutpopulations in Høylandet catchment, a atmosphaericlow deposition area in Mid Norway. There was asignificant inverse correlation in abundance ofepibenthic Arctic charr and brown trout in theselakes, indicating that the latter species is dominant.The largest numbers of sympatric brown trout andArctic charr were caught in epibenthic habitat. In twolakes, brown trout to some extent also occurredpelagically, while pelagic individuals of Arctic charrwere found in all five lakes. The main food items forboth epibenthic and pelagic brown trout wereterrestrial surface insects and chironomid pupae.Zooplankton was the primary food item for Arctic charrin both habitats. Although the age distribution wasvery different in the populations, neither speciesseem to suffer from recruitment failure. There was nosignificant difference in survival rates betweensympatric populations of brown trout and Arctic charr.We found a significant inverse correlation betweenepibenthic catches of brown trout and the mean weightof 4+ fish, the most abundant age group. However, ifusing weight data for three-year-old fish, no suchrelationship was found for Arctic charr. Brown troutand Arctic charr reached asymptotic lengths of197–364 mm and 259–321 mm, respectively. Both speciestypically reached sexual maturity at age 2–3, and nomaturation-induced mortality was evident. We concludethat fish populations in Høylandet lakes areregulated throughout their lifes by inter- andintraspecific competition.     

Demographic and environmental influences on life-history traits of isolated populations of the Andean catfish Astroblepus ubidiai

In this study, we examined the influence of demographic and environmental variables on the life histories of six remnant populations of the Andean catfishAstroblepus ubidiai (Actinopterygii; Siluriform) located in isolated refuges, and tested six predictions on the relationships between age at maturity, generation time, population density and juvenile and adult growth and survival rates. The three populations inhabiting the watershed with a direct connection to a major lake (Imbakucha) exhibited later maturity, a longer generation time, higher adult survivorship, a higher adult-to-juvenile survival ratio and lower biomass density than those of the adjacent, non-lacustrine watersheds. Across all six populations, there was a strong correlation between mean age at maturity and both the adult-to-juvenile survival ratio and biomass density. Study populations also showed a pattern of inverse density-dependence associated with Allee effects. We conclude that without limitations in the availability of environmental resources, higher population density can be negatively related to age at maturity. This relationship would be expected to become positive when carrying capacity of the systems is reached. Delayed maturity in the Imbakucha populations may be an adaptive response for increasing juvenile survival under stressful conditions.     

Differences in size and growth rates of male and female migrating Japanese eels in Pearl River, China

The Sr/Ca ratios in otoliths of silver Japanese eels Anguilla japonica , in Pearl River, China, indicated that both sexes did not stay in brackish water and grew in fresh water from the glass eel stage until spawning migration. This did not support the hypothesis that females tended to distribute upstream and males might be restricted to estuaries. The back-calculated total length of males at glass eel stage was not significantly different from that of females, indicating that the hypothesis that small glass eels became males and larger ones became females may not be true. The mean (±S.D.) age and total length of males at migration were 6·4±1·6 years and 48·3±4·5 cm, which were significantly smaller than for females, 8·3±1·6 years and 61·4±4·1 cm. The age of migration was related inversely to growth rate for both sexes. Growth parameters of the von Bertalanffy growth equation were K =0·21 cm year°1, L _∞=55·7 cm and t _o=-0·55 year for males and K =0·14 cm year⁻¹, L _∞=77·5 cm and t _o=-0·60 year for females. The difference in asymptotic length ( L _∞) between males and females may be because females postpone migration to achieve larger size for maximizing reproductive success.     

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.     

Relationships of reproductive traits and body size with attainment of sexual maturity and age in Blanding's turtles (Emydoidea blandingi)

To place associations among body size, age at maturity, age, and reproductive traits of a long-lived organism in the context of current life history models based on the concept of norms of reaction, we examined data from a mark-recapture study of Blanding's turtles (Emydoidea blandingi) in southeastern Michigan during 24 of the years between 1953 and 1988. Females matured between 14 and 20 years of age. Both the smallest and largest adult females in the population were reproducing for the first time in their lives. This result suggests that a combination of differences in juvenile growth rates and ages at maturity, and not indeterminate growth, are the primary cause of variation in body size among adults. Body size variation among individuals was not related to age at sexual maturity. Females that had slower growth rates as juveniles matured later at similar mean body size compared to those with more rapid growth that matured at an earlier age. As a result, a linear model of age at sexual maturity with growth rates of primiparous females between hatching and maturity was significant and negative (R² = 0.76). Frequency of reproduction of the largest and smallest females was not significantly different. Clutch size did not vary significantly with age among either primiparous or multiparous females. Clutch sizes of primiparous females and multiparous females were not significantly different. However, older females (>55 years minimum age) reproduced more frequently than did younger females (minimum age <36 y).     

The relative abundance of brown trout in acidic softwater lakes in relation to water quality in tributary streams

The influence of the water quality of tributary streams on the relative abundance in benthic gillnet catches (catch per unit effort, cpue) of allopatric brown Salmo trutta was assessed in associated acidic, softwater lakes. The study was carried out over 6 years (1989–1994) in 15 lakes located at altitudes between 230–715 m a.s.l. in two Norwegian catchments. The water quality of the main inlets and outlets varied little, as indicated by their of pH range (4·93–5·51) and calcium concentrations (0·19–0·44 mg 1⁻¹), but varied more with respect to concentrations of inorganic, monomeric aluminium (7·0–41·0 μg l⁻¹). Most of the lakes were also fed by secondary streams with better water quality: a maximum pH of 6·56, calcium levels of up to 0·74mg 1⁻¹, and inorganic aluminium levels as low as l·0 μg 1⁻¹. The cpue was inversely correlated with lake altitude ( r ²=0·50), and thus was adjusted to a mean altitude. The calcium concentration in the richest secondary stream to each lake, its richness judged on the basis of its acid-neutralizing capacity, had the highest predictive power of the variability in cpue ( r ² = 0·49).The calcium content in the other secondary streams or in the main inlets and outlets did not correlate with cpue. Alkalinity in the main outlets correlated to some extent with cpue ( r ² = 0·27). It is suggested that secondary streams with good water quality provide important refuges for the recruitment of brown trout in acidic softwater lakes.     

Reproductive biology, length-weight relationship and relative condition of pond raised Tilapia zilli (Gervais)

In 'runted' populations of Tilapia zilli positive correlations were found between maturation stages and the following: gonadosomatic index, gonad weight, fish weight and fish length. The minimum size at maturity was 9·0 cm in males and 11·0 cm in females. The mean fecundity in the 'stunted' females was 2359 eggs and it increased with length ( L ) weight (HO and depth ( D ) of the fish. There was, however, no correlation between fecundity and egg size. The results indicated that growth in this 'stunted' T. zilli population was not isometric. The relative condition factor ( Kn ) which approached one indicates good condition.     

Life-history correlates of maximum population growth rates in marine fishes

Theory predicts that populations of animals with late maturity, low fecundity, large body size and low body growth rates will have low potential rates of population increase at low abundance. If this is true, then these traits may be used to predict the intrinsic rate of increase for species or populations, as well as extinction risks. We used life-history and population data for 63 stocks of commercially exploited fish species from the northeast Atlantic to test relationships between life-history parameters and the rate of population increase at low abundance. We used cross-taxonomic analyses among stocks and among species, and analyses that accounted for phylogenetic relationships. These analyses confirmed that large-bodied, slow-growing stocks and species had significantly lower rates of recruitment and adult production per spawning adult at low abundance. Furthermore, high ages at maturity were significantly correlated with low maximum recruit production. Contrary to expectation, fecundity was significantly negatively related to recruit production, due to its positive relationship with maximum body size. Our results support theoretical predictions, and suggest that a simply measured life-history parameter can provide a useful tool for predicting rates of recovery from low population abundance.     

Cumulative effect of the North Atlantic Oscillation on age‐class abundance of albacore (Thunnus alalunga)

The aim of this study was to look into possible relationships between climate and the inter‐annual variability of albacore (Thunnus alalunga) catch rates by age class observed during a recreational fishery tournament at the spawning grounds of S’Estanyol (Balearic Islands, Spain) in the years 2004–2009. The mean capture per unit effort (CPUE) was significantly higher when the mean of the North Atlantic Oscillation (NAO) experienced by the albacore in winter and spring of its life history (NAO_life) was negative than when the NAO_life was positive. A statistically significant negative relationship was obtained between NAO_life and the probability of a CPUE value for an age class and year being higher than the average CPUE for all the cohorts in that age class. The results suggest that local abundance of albacore in a spawning grounds could be related to environmental factors dependent on the NAO and that there may be a cohort‐age effect. It is hypothesized that factors dependent on the NAO, such as the abundance variation of small pelagic fishes as a response to the NAO variability, could have a cumulative effect over the good biological condition (fitness) of a long‐living fish predator such as the albacore.     

Age and size-dependent variation in the seasonal timing and probability of reproduction among mature female pumpkinseed,Lepomis gibbosus

Synopsis In this study, we used pumpkinseed,Lepomis gibbosus, populations in two east-central Ontario lakes to test for age and size-dependent effects on the probability and timing of reproduction of mature females. Pumpkinseed body size characteristics differed in the two lakes; Little Round Lake harbours a stunted population and Beloporine Lake does not. Age-class distribution of mature females was determined by a maturity assessment on fish collected just prior to spawning and at the mid-spawning period, combined with an early-season capture-mark-recapture survey. Spawning females were collected throughout the breeding season to assess age and size-related temporal trends, and to compare their age and size distribution with that of mature females at large. The proportion of age 2 females in Little Round Lake that actually spawned was considerably lower than the proportion of mature age 2 females at large. Furthermore, age 2 females that spawned in this lake did not do so until late in the breeding season. In contrast, the proportion of young/small females spawning in Beloporine Lake was comparable to the proportion of young/small mature females at large, and both small and large females spawned throughout the breeding season. Small mature females in Little Round Lake may have had no other option but to spawn late in the season because of their poor body condition. In Beloporine Lake, condition factor early in the breeding season in age 2–4 females was higher than that of Little Round Lake females, suggesting that limited energy reserves in the spring may have prevented young Little Round Lake females from early spawning. Our results show that the likelihood and timing of reproduction are both age and size-dependent in some populations. Small individuals that delay seasonal maturation and spawn late in the summer probably contribute little to the population due to the restricted growth and reduced overwinter survival of their progeny.To whom correspondence should be addressed