Female mate choice and spawning behaviour of Chinook salmon under experimental conditions

After spawning their first nest, female chinock salmon Oncorhynchus tshawytscha paired with small males ( c. 46% of female weight) spent an average of 16·2 h between spawning of successive nests compared with 9·6 h for females paired with large males ( c. 112% of female weight) ( P< 0·05). Neither frequencies of female nest construction behaviours (digging and probing) nor male courtship behaviour (crossovers and quivers) differed between large- and small-male pairs. Male quivering frequencies were correlated significantly with female digging and probing frequencies, whereas the crossover frequencies were not. It is suggested that delayed spawning by females in the presence of relatively small males is a primary mechanism by which females in the genus Oncorhynchus exhibit mate choice.     

Dynamic changes in prey choice by stickleback during simultaneous encounter with large prey

When a three-spined stickleback Gasterosteus aculeatus encountered prey simultaneously the probability of hanging and the median pursuit time were greater than when prey were encountered sequentially. During simultaneous prey encounter fish did not choose to attack the more profitable prey but instead the nearer prey was handled first except when the difference between the two prey sizes was large. No difference was found in the level of total energetic intake by the fish regardless of prey size pairing. Fish that handled and ate the first prey of a pair in <5 s attacked the second prey with a high probability of success, demonstrating an opportunistic feeding strategy. Importantly however, the fish did not choose to maximize long term energy intake rate by eating both prey, but rather short-term considerations over the course of feeding took precedence. With an empty stomach, the probability of a fish eating ( P _eat) the first prey handled was high regardless of prey size. As stomach fullness increased, the P _eat the first prey handled decreased if it was the larger prey. Hence, the fish were unselective when the stomach was empty but thereafter there was a shift in preference towards the smaller prey. The decision of which prey to attack and eat appeared to be based on short-term energy considerations and the level of stomach fullness. This study demonstrates that feeding on a short-term scale is a crucial factor to take account of when analysing fish feeding during simultaneous prey encounter.     

Daily feeding rhythms in Atlantic salmon II: size-related variation in feeding patterns of post-smolts under constant environmental conditions

Previous studies of feeding rhythms in post-smolt Atlantic salmon Salmo salar have produced variable results. Therefore, the present study was performed to clarify the source of these differences and to examine size-related variation in daily feeding patterns. A small group of hatchery reared post-smolts was kept in an indoor seawater tank under conditions of constant photoperiod and light intensity, and negligible diurnal variation in water temperature. Feeding responses were monitored by video camera throughout the day. The resultant feeding pattern for the population mirrored an earlier study, with the highest levels of feeding activity in the early morning and late evening. Differences between this and other studies may be due to variation in climate, season or life-history stage. Daily feeding patterns were size-dependent. The largest fish fed at a relatively constant level through the day, medium-sized fish fed in a pattern similar to that described above for the population and small fish fed most at first and last light. These differences were due probably to the effect of competitive interactions upon fish of different sizes.     

Daily feeding rhythms in Atlantic salmon I: feeding and aggression in parr under ambient environmental conditions

Daily feeding patterns of tank-reared Atlantic salmon Salmo salar parr under ambient conditions and their relationship with patterns of aggression were investigated during spring and autumn. A single, early afternoon feeding peak was observed during spring but no consistent pattern emerged from the autumn trial (fish did tend to feed in synchrony, even though the population's feeding pattern varied between days). Aggression was highest at first light during both spring and autumn, reducing thereafter throughout the day. Feeding and aggression were out of phase during spring, with the level of feeding increasing as aggression decreased. It is suggested that the high levels of early morning aggression are associated with re-establishment of the social order that breaks down after a period of darkness; in the spring this may have inhibited feeding.     

Individual feeding success of juvenile perch is consistent over time in aquaria and under farming conditions

Juvenile (0+ year) perch Perca fluviatilis , held in tanks (1 m³) for 8 months under constant temperature (17° C) and light (12L : 12D) conditions, were weighed monthly. Individually marked fish from the tanks were placed in aquaria in groups of four and tested for feeding success, assessed as individual share of a group meal in three identical short‐term aquarium experiments separated in time (January, May and June). The groups consisted of the same individuals in each of the three experiments. In tanks, growth (thermal growth coefficient, G _T) was used as a measurement of feeding success. Individual G _T in tanks was consistent over time, and individual share of a group meal in aquaria was consistent between consecutive months. There was no connection between individual feeding success in aquaria and G _T in tanks in May and June, and in January the factors were negatively correlated. The condition factor at the start of aquarium experiments and feeding success in aquaria were negatively correlated in June and tended to be so also in May. Boldness in aquaria was negatively correlated with growth in tanks. Thus, individuals that were successful in one environment seemed to be less successful in the other. There was a seasonal change in G _T, with an increase from March to April and from April to June despite constant day length and temperature.     

Female preference for preferred males is reversed under low oxygen conditions in the common goby (Pomatoschistus microps)

Female preference for males that already have eggs in theirnest has been reported in many fish species. The presence ofeggs may provide a cue for copying the choice of previous femalesand may indicate that high-quality care will be available. Consistentwith a review of 13 studies, we found that female common gobies(Pomatoschistus microps) preferred to spawn with males thathad already been chosen by females and whose nests therefore alreadycontained eggs. However, this preference was reversed underconditions of low dissolved oxygen. We would not expect thisreversal if the second female were using eggs as a signal ofmale genetic attractiveness to other females unless the benefitswere outweighed by direct selection. The reversal also couldnot be explained by differences in active courtship by males,as courtship rates did not differ under low oxygen between maleswith or without eggs. Low oxygen conditions corresponded witha nearly threefold increase in male ventilation of eggs anda reduction in time spent near a selecting female. The reversalis therefore most likely due to females avoiding males thatwould be unable to meet the demands of care of a second clutchunder low oxygen conditions. Thus, an abiotic feature of theenvironment reveals plasticity of female choice, consistentwith hypothesized changes in benefits of mating with preferredmales.