Growth parameters and total mortality in Oreochromis niloticus (Linnaeus) from Nyanza Gulf,Lake Victoria

Length-frequency data collected from fish landings in the Kenya waters of Lake Victoria were used to estimate the growth parameters, total mortality rate and growth performance index in Oreochromis niloticus. The asymptotic length, (L _∞) and the ratio of the total mortality rate (Z) to the growth constant (K), were estimated to be 64.6 cm and 3.219 respectively. K was 0.254 y^-1, Z was 0.818 y^-1 and the growth performance index θ′ = Log₁₀ K + 2 log₁₀ L∞ = 3.025, which is rather high as compared to other tilapia populations in natural waters.     

The fisheries biology of flying fishes (Families: Exocoetidae and Hemiramphidae) from the Camotes Sea, Central Philippines

The catches of a small artisanal fishery for flying fishes (Families Exocoetidae and Hemiramphidae) on the Danajon Bank in the Camotes Sea. Central Visayas, were recorded during a 14-month period between 1987–1988. Catches were made using floating drive-in-nets deployed from small motorized canoes. Three species, Cheilopogon nigricans, Cypselurus opisthopus and Oxyporhamphus convexus , formed about 90% of landings. Growth, mortality and related parameters for the three dominant species in the catch were estimated from length-frequency data. Seasonal variations in catch rate and recruitment are described and thought to be linked to the two monsoon periods in the Philippines. Total mortality rates were very high and, while these may be the result of migratory movements rather than attrition, they are a cause for concern in such a highly selective fishery.     

On the Population Ecology of the Zebra Seabream Diplodus cervinus cervinus (Lowe 1838) from the Coasts of the Canarian archipelago, North West Africa

Zebra seabream, Diplodus cervinus cervinus, caught off the Canary Islands is characterized by protogynous hermaphroditism. The male:female ratio is in favour of females (1:2.16). The reproductive season extends from spring to summer, with a peak in spawning activity in May–June. Males reach maturity at a larger total length, 327mm (5 years old) than females 273mm (4 years old). Recruitment occurs from late October to January in shallow waters of 0.5–8m depth along the coastal line. The recruits are located over rocky substrates with an important algae vegetation forming schools lower than 2m². During the spawning season, schools of adults from 3 to 8 fish are observed. The schools are formed by one large individual and a few moderate size individuals. Mating takes place in small groups formed by one dominant male and a group of several females (polygamy). Moderate size individuals are often observed mixed in large schools (up to 30 individuals) of Diplodus sargus cadenati. Subadults form groups of a few fish (<5 individuals) or more commonly mixed groups (>15 individuals) with individuals of species of similar size. Otoliths age readings indicate that the population consists of 18 age groups, including a very high proportion of individuals between 2 and 4 years old. The von Bertalanffy growth parameters for the whole population are: L = 603mm, k = 0.149 year^–1, and t₀ = –0.22 year. The mean rates of total, natural and fishing mortality are 0.551, 0.215 and 0.336 year^–1, respectively. The length at first capture is 183mm. The exploitation rate indicates that the stock is overfished. The direct effects of fishing on the population result in changes in the abundance, with a reduction to 85% of the unexploited equilibrium level. The length at first capture by the commercial fishery is less than the length at maturity. With 58% of the total catch below this length there is a danger of recruitment overfishing.     

Fish introductions reveal the temperature dependence of species interactions

A major area of current research is to understand how climate change will impact species interactions and ultimately biodiversity. A variety of environmental conditions are rapidly changing owing to climate warming, and these conditions often affect both the strength and outcome of species interactions. We used fish distributions and replicated fish introductions to investigate environmental conditions influencing the coexistence of two fishes in Swedish lakes: brown trout (Salmo trutta) and pike (Esox lucius). A logistic regression model of brown trout and pike coexistence showed that these species coexist in large lakes (more than 4.5 km²), but not in small, warm lakes (annual air temperature more than 0.9–1.5°C). We then explored how climate change will alter coexistence by substituting climate scenarios for 2091–2100 into our model. The model predicts that brown trout will be extirpated from approximately half of the lakes where they presently coexist with pike and from nearly all 9100 lakes where pike are predicted to invade. Context dependency was critical for understanding pike–brown trout interactions, and, given the widespread occurrence of context-dependent species interactions, this aspect will probably be critical for accurately predicting climate impacts on biodiversity.     

Natural 'poor start' does not increase mortality over the lifetime

Poor nutrition and other challenges during infancy can impose delayed costs, and it has been proposed that expression of costs during adulthood should involve increased mortality rather than reduced reproduction. Demonstrations of delayed costs come mostly from experimental manipulations of the diet and hormones of captive infants of short-lived species, and we know very little about how natural poor starts in life affect wild animals over their lifetimes. In the blue-footed booby, sibling conflict obliges younger brood members to grow up suffering aggressive subordination, food deprivation and elevated stress hormone, but surviving fledglings showed no deficit in reproduction over the first 5-10 years. A study of 7927 individuals from two-fledgling and singleton broods from 20 cohorts found no significant evidence of a higher rate of mortality nor a lower rate of recruitment in younger fledglings than in elder fledglings or singletons at any age over the 20 year lifespan. Development of boobies may be buffered against the three challenges of subordination. Experimental challenges to neonates that result in delayed costs have usually been more severe, more prolonged and more abruptly suspended, and it is unclear which natural situations they mimic.     

Patterns of natural mortality in stream‐living brown trout (Salmo trutta)

1. We tested the hypothesis that lifetime mortality patterns and their corresponding rates and causal factors differ among populations of stream‐living salmonids. To this end, we examined the lifetime mortality patterns of several successive cohorts of two stream‐living brown trout (Salmo trutta) populations in Spain and Denmark. 2. In the southern population, we observed a consistent two‐phase pattern, in which mortality was negligible during the first half of the lifetime and severe during the rest of the lifetime. In contrast, the northern population demonstrated a three‐phase pattern with an earlier phase varying from negligible to severe, followed by a second stage of weak mortality, and lastly by a third life stage of severe mortality. 3. Despite substantial differences in the mortality patterns between the two populations, the combined effect of recruitment (as a proxy of the density‐dependent processes occurring during the lifetime) and mean body mass (as a proxy of growth experienced by individuals in a given cohort) explained c. 89% of the total lifetime mortality rates across cohorts and populations. 4. A comparison with other published data on populations of stream‐living brown trout within its native range highlighted lifetime mortality patterns of one, two, three and four phases, but also suggested that common patterns may occur in populations that experience similar individual growth and population density.