Response of upstream migrating juvenile European eel (Anguilla anguilla) to electric fields: Application of the marginal gains concept to fish screening.

The decline in European eel (Anguilla anguilla) recruitment over the past half-century is partly due to river infrastructure that delays or blocks upstream migration to rearing habitat. Stimuli, such as electricity, can be used to modify the behaviour of downstream moving fish and guide them to preferred routes of passage at river infrastructure; but research on upstream migrating juvenile eel remains limited. The response of upstream migrating juvenile eel exposed to pulsed direct current (PDC) electric fields was investigated using a recirculatory flume. Eel were presented a choice of two routes upstream under either: (1) a treatment condition, in which the selection of one route resulted in exposure to High Electric Field (HEF) strength that was between 1.5-2 times stronger than the Low Electric Field (LEF) strength encountered in the alternative route; or (2) a control in which the electric field was absent in both routes. Under the treatment, five different mean HEF strengths (0.53, 0.77, 1.22, 2.17 and 3.74 Vcm-1) were tested at one of two frequencies (2 and 10 Hz). Route choice, distance downstream of the first set of electrodes at which an initial response was observed and avoidance behaviours (acceleration, retraction, switching and rejection) were compared among treatments. For the 1.22, 2.17 and 3.74 Vcm-1 and under 2 Hz, eel preferred to pass the LEF route. Avoidance was greater in the HEF route and positively related to field strength. The distance of the initial response did not differ between routes, field strengths or frequency. Upstream migrating eel avoided electric fields indicating potential to develop this approach for fish guidance. Further work is needed to test prototypes in field settings, particularly in combination with traditional physical screens to water intakes as part of a process of applying the concept of marginal gains to advance environmental impact mitigation technology.

The rationality of decisions depends on behavioural context.

Decision makers can be described as economically rational (making consistent choices), or economically irrational (making choices that vary with the options available). As the extent to which animals can and do make rational versus irrational decisions remains unclear, we tested the decision-making strategies of female Nasonia vitripennis parasitic wasps in two behavioural contexts: oviposition and foraging. In our first experiment, to determine whether oviposition preferences changed depending on the options available, we presented females with a high and a medium-quality blow fly host to parasitize, and gave some females an additional low or very low quality 'decoy' host. Presence of decoy options did not affect females' oviposition choices, either in willingness to parasitize a host or the number of offspring laid. In our second experiment, we tested the effects of a low-quality decoy option on foraging preference for a high and a medium-quality sucrose concentration option. Here, presence of the low-quality decoy enhanced female preference for the high-quality option. Females therefore made economically rational decisions when ovipositing and economically irrational decisions when foraging. This difference in decision outcomes suggests that the cost/benefit ratio of making one type of decision over another may differ with the behavioural task.

Interplay between age-based competitive asymmetries within the brood and direct competition between inbred and outbred offspring in a burying beetle.

Theory suggests that intraspecific competition associated with direct competition between inbred and outbred individuals should be an important determinant of the severity of inbreeding depression. The reason is that, if outbred individuals are stronger competitors than inbred ones, direct competition should have a disproportionate effect on the fitness of inbred individuals. However, an individual's competitive ability is not only determined by its inbreeding status but also by competitive asymmetries that are independent of an individual's inbreeding status. When this is the case, such competitive asymmetries may shape the outcome of direct competition between inbred and outbred individuals. Here, we investigate the interface between age-based competitive asymmetries within broods and direct competition between inbred and outbred offspring in the burying beetle Nicrophorus vespilloides. We found that inbred offspring had lower survival than outbred ones confirming that there was inbreeding depression. Furthermore, seniors (older larvae) grew to a larger size and had higher survival than juniors (younger larvae), confirming that there were age-based competitive asymmetries. Nevertheless, there was no evidence that direct competition between inbred and outbred larvae exacerbated inbreeding depression, no evidence that inbreeding depression was more severe in juniors and no evidence that inbred juniors suffered disproportionately due to competition from outbred seniors. Our results suggest that direct competition between inbred and outbred individuals does not necessarily exacerbate inbreeding depression and that inbred individuals are not always more sensitive to poor and stressful conditions than outbred ones.