Delayed maturity does not offset negative impact afflicted by ectoparasitism in salmon

The negative effects of parasitism on host population dynamics may be mediated by plastic compensatory life-history changes in hosts. Theory predicts that hosts should shift their life-history towards early reproduction in response to virulent pathogens to maximize reproduction before death. However, for sublethal infections that affect growth, hosts whose fecundity is correlated with body size are predicted to shift towards delayed reproduction associated with larger body size and higher fecundity. This has been observed in Atlantic salmon and parasitic sea lice, via mark-recapture studies that recover mature fish from paired groups of control and parasiticide-treated smolts. We investigated whether such louse-induced changes to age at maturity can offset some of the negative effect of mortality on population growth rate in salmon using a structured population matrix model. Model results show that delayed maturity can partially compensate for reduced survival. However, this only occurs when marine survival is moderate to poor and growth conditions at sea are good. Also, the impact of delayed maturity on population growth when parameterizing the model with empirical data is negligible compared with effects of direct mortality. Our model thus suggests that management that works on minimizing the effect of sea lice from fish farms on wild salmon should focus mainly on correctly quantifying the effect of parasite-induced mortality during the smolt stage if the goal is to maximize population growth rate.publishedVersio

Making the best of lousy circumstances: The impact of salmon louse Lepeophtheirus salmonis on depth preference of sea trout Salmo trutta

Sea trout are known for seeking out sources of freshwater to rid themselves of salmon lice. However, the effect of natural haloclines in fjords on parasite dynamics is not well understood. We tagged 48 naturally infested wild sea trout with acoustic depth sensors. The fish were kept inside a small net-pen (4 × 4 × 5 m), 12 at a time, in western Norway during 4 separate time periods in spring 2017. The sea trout were relatively highly infested with sea lice (prevalence: 100%, mean ± SD: 68 ± 58 lice fish-1), and a relatively large proportion of the individuals did not survive the trials (25% mortality). The results show that temperature and light were the 2 most important factors explaining the vertical behaviour of the surviving trout. Mobile lice also had a significant effect on depth distribution, where fish with higher abundances of lice were observed at shallower depths. During the 7 d periods in the net-pen, total sea louse abundance decreased from a mean of 68 to 35 fish-1. Surface salinity explained this reduction better than the experienced salinity of the individual fish, suggesting that short-time exposure to very low salinities, rather than long-term exposure to moderate salinities, is the driving force behind the effect of haloclines on reduction in sea lice numbers.publishedVersio

Predation by Eurasian otters on adult Atlantic salmon

The return of the Eurasian otter to western Norway has sparked human-predator conflicts as otters prey on vulnerable Atlantic salmon populations. Although predation may not be the direct cause of salmon population declines, otters that kill salmon in rivers before they spawn may impact the salmon spawning stock, with potential consequences for stock recruitment. Concerns of local people and stakeholders suggest that otter predation inhibits recovery of salmon populations. However, there is limited information on mortality caused by otter predation on adult salmon. To gain insight into impacts of otter predation on salmon populations, I quantified the predation by otters on adult salmon in two rivers in Western Norway using a novel combination of radiotelemetry and temperature loggers. I tagged 45 salmon in Aureelva and 30 salmon in Søre Vartdalselva and tracked the salmon until they died or left the river. This method identified the fates of 95 % of tagged salmon. Otters killed 9 tagged salmon in Aureelva and 20 tagged salmon in Søre Vartdalselva. I found no evidence that otters selectively killed salmon based on sex, length, health status or activity level, which suggests that predation mortality on pre-spawners was additive. Otter predation contributed in reducing both salmon populations below their spawning targets, and without otter predation both populations would have been closer to reaching their spawning targets. However, the magnitude of predation differed greatly between rivers. Salmon in Søre Vartdalselva had greater predation risk compared to salmon in Aureelva, possibly due to differences in the number of holding pools between the two rivers. The findings from this study emphasise that management decisions should be guided by river-specific evaluations of impacts of otter predation on salmon, for which the combination of radiotelemetry and temperature loggers can provide a valuable tool.Predation by Eurasian otters on adult Atlantic salmonpublishedVersio

Assessing the occurrence of egg stranding for trout and salmon in a regulated river

A key challenge in many regulated rivers is to define adequate flow levels to protect aquatic organisms. Provisioning of suitable flow can be pivotal bottlenecks for fishes such as salmon and trout that use the riverbed as an incubation habitat. Additionally, the locations where females spawn will define the probability that embryos will be dewatered during the incubation period in a regulated flow regime. We investigated the water flow, dewatering, and incubation mortality in Atlantic salmon and brown trout in natural nests over 19 years in the regulated Bjoreio River in western Norway. During the study period, different flow strategies were applied to mitigate the dewatering of incubating salmon and trout embryos. Average survival in nests sampled in late winter ranged from 54% to 92% among years and was significantly correlated with the minimum water flow occurring during the incubation period. Mortality was significantly higher in nests in shallow areas, reflecting nests exposed to dewatering. The results demonstrate a strong link between incubation mortality and managed flow regimes for river spawning salmonids. Using detailed information on the nest location and incubation mortality, we estimate minimum flow requirements for this river and demonstrate an approach to effectively mitigate the impact of river regulations on embryo survival in Atlantic salmon and brown trout.publishedVersio

¿Qué pueden decirnos las distribuciones de talla dentro de cohortes sobre los procesos ecológicos en larvas de peces?

Marine fish larvae are subject to variable environments, which is probably reflected in their growth and survival rates. Mortality rates are generally high and size-dependent. At the species level, these mortality rates are usually accompanied by correspondingly high growth rates. Here we provide examples from experimental studies with Atlantic cod (Gadus morhua) and Atlantic herring (Clupea harengus) larvae, in which multiple cohorts were followed over time. Body size, prey concentrations, and temperature are shown to influence growth rates. We present a method based on cumulative size distributions (CSDs) for visualizing variability of sizes within cohorts over time. Analysis of CSDs revealed size-selective mortality and variations among populations in size- and temperature-dependent growth throughout ontogeny. We found that cod larvae consistently exhibit higher growth rates than herring larvae. While cod larvae may have an advantage over herring larvae when food availability is high, herring were more able to survive at low food concentrations than cod. Cod and herring seem to represent two growth strategies: cod larvae are relatively small at hatching and a high growth rate appears to be a prerequisite for success, whereas herring larvae are initially large, but grow more slowly.Las larvas de peces marinos están sujetas a ambientes variables que probablemente se reflejan en sus tasas de crecimiento y supervivencia. las tasas de mortalidad son generalmente altas y dependientes de la talla. A nivel de especies, estas tasas de mortalidad están usualmente acompañadas de tasas de crecimiento altas. en este trabajo mostramos ejemplos a partir de estudios experimentales con larvas de bacalao atlántico (Gadus morhua) y arenque atlántico (Clupea harengus), en los que se siguieron cohortes múltiples a lo largo del tiempo. Se muestra como la talla del cuerpo, la concentración de presas y la temperatura influyen en la tasa de crecimiento. Presentamos un método basado en distribuciones de frecuencias de talla acumuladas (DTAs) para visualizar la variabilidad en tallas dentro de las cohortes a lo largo del tiempo. el análisis de DTAs reveló mortalidad selectiva por talla, y variaciones entre poblaciones en el crecimiento dependiente de la talla y la temperatura a través de la ontogenia. encontramos que las larvas de bacalao mostraron consistentemente mayores tasas de crecimiento que las de arenque. Mientras las larvas de bacalao pueden tener una ventaja sobre las de arenque cuando la disponibilidad de presas es alta, las de arenque son más capaces de sobrevivir a bajas concentraciones de comida. Bacalao y arenque parecen representar dos estrategias de crecimiento; las larvas de bacalao son relativamente pequeñas a la eclosión y una alta tasa de crecimiento parece un prerrequisito para el éxito, mientras que las de arenque son inicialmente más largas, pero crecen más lentamente

Moving cleaner fish from the wild into fish farms: A zero-sum game?

Fish that engage in mutualistic cleaning behaviour (‘cleaner fishes’) have recently been popularized as a nature-based method of controlling ectoparasite outbreaks in fish farms. Outbreaks impact animal welfare and threaten wild fish populations; due to this, millions of cleaner fish (especially wrasses from the family Labridae) are wild-caught each year and transferred into Atlantic salmon (Salmo salar) farms to remove ectoparasitic sea lice (predominantly Lepeophtheirus salmonis) from farmed salmon and reduce spillover (i.e. parasites transferring from farmed to wild fish) onto vulnerable wild salmonid populations. However, we hypothesize that this practice may result in no net benefit to the infestation pressure on wild fish if gains in farm-based control trade off against the removal of lice from wild fish by wrasse moved from the wild into net pens. Such a scenario would entail a zero-sum game. We test our hypothesis using an ecological simulation of wrasse as cleaners of farmed Atlantic salmon and wild sea trout (Salmo trutta). We parameterized our simulation based on published models of sea lice epidemics from farms to calculate the relative impact of lice when wrasse are removed from the ecosystem to serve as cleaners. Our simulations revealed that a zero-sum game can emerge from this system at unexpectedly infrequent rates of cleaning by wrasse in the wild. Scandinavian wrasses are relatively data poor and parameterizing our simulation revealed a need for better data on the ecological role of these fishes in coastal ecosystems. For the first time, we suggest that wrasse fisheries may be a zero-sum game and that under a plausible set of conditions, fishing wrasse out of coastal ecosystems may do more harm than good for modulating sea lice epidemics. However, we emphasize that these results do not suggest that wrasse alone will play a role in resolving high infestation pressure of lice emanating from fish farms.publishedVersio

Estimating the temporal overlap between post-smolt migration of Atlantic salmon and salmon lice infestation pressure from fish farms

To be able to design effective management to alleviate wild fish from parasite infestation pressure from fish farms, it is pivotal to understand when post-smolts migrate past areas of potential exposure to salmon lice Lepeophtheirus salmonis. Here, data from release groups of coded-wire-tagged Atlantic salmon Salmo salar smolts and their subsequent recaptures in a trap net in the outer fjord 12 to 97 km from the various release sites were used to estimate the smolts’ progression rate and their arrival time in an outer fjord in Norway. The arrival time estimates to the outer fjord are compared with modelled infestation pressure from local fish farms. The overall progression rate varied from 0.8 to 31.2 km d-1 (0.05 to 2.20 body lengths s-1), with mean and median values of 8.8 and 7.8 km d-1, respectively (0.60 and 0.54 body lengths s-1). The progression rate varied with water discharge from the rivers into the fjords, fish length, condition factor and smolt origin. Simulated arrival time and capture of wild smolts suggest that smolts from the different rivers arrive in the outer fjord system with a difference of up to 4 wk. The arrival time for the rivers with the longest migration was estimated to be from mid-May throughout June. Infestation pressure from fish farms increased from the beginning of June in 2 of 3 study years, suggesting that an increase in lice exposure from fish farms will overlap with smolts from late-migrating populations in some but not all years

Habitatkartlegging i Strynselva høsten 2020

Denne rapporten sammenstiller resultater av habitatkartlegging utført av NORCE LFI i Strynselva i Stryn kommune høsten 2020. Kartleggingen viser at mangel på gytehabitat og skjul er viktige habitatflaskehalser. Det anbefales habitat-/eller restaureringstiltak som vil øke tilgjengelig gyteareal og oppvekstareal for laks og sjøørret.publishedVersio

Effects of tag type and surgery on migration of Atlantic salmon (Salmo salar) smolts

Tagging salmon smolts to provide information about the timing of outmigration has been a common approach to monitor phenology and model the risk of encountering stressors. However, the validity of tagging has come under scrutiny because of the sensitivity of this parameter in various management systems. We studied the probability of migration, timing of migration and growth during migration for Atlantic salmon smolts tagged with three different tags in the River Dale, western Norway. Two groups were tagged with passive integrated transponder (PIT) tags via a small ventral nonsurgical incision, either a 12 mm or a new 16 mm PIT tag. Two groups were subjected to surgical implantation of either a dummy acoustic transmitter or a 12 mm PIT tag (a sham surgery). Overall, 71% of the tagged smolts were recaptured at the downstream Wolf trap. Smolts from the sham tagged group were recaptured most frequently (78%) compared to dummy acoustic transmitters and 16 mm PIT tags (both 68%), but the differences were not significant. Results agree with prior assessments that longer smolts migrated earlier, with about half a day earlier migration for each millimetre total length of the smolt, but did not suggest any difference in time of migration among the tag types. Growth in length was evident from release to recapture, with smaller smolts exhibiting greater growth and no effect of tagging treatment. Our findings suggest that inferences about the timing of outmigration for salmon smolts based on acoustic tagging should be made cautiously because of the relationship among tag size, suitable fish size and the timing of a tagged individual's migration.publishedVersio

Predation research with electronic tagging

Predation is a fundamental aspect of ecology that drives ecosystem structure and function. A better understanding of predation can be facilitated by using electronic tags that log or transmit positions of predator or prey species in natural settings, however, there are special considerations that must be made to avoid biased estimates. We provide an overview of the tools available for studying predation with electronic tags including the tag types and analytical tools that can be used to identify where, when and how prey are killed by predators. We also discuss considerations for experimental design when studying predation using electronic tags, including how to minimize effects of capture and tagging procedures. Ongoing innovation and integration of sensors for tags will provide more detailed data about the performance of tagged predators and the fate of tagged prey. Where analysts can effectively resolve the timing of predation using state-of-the-art tags and analytical tools, we foresee exciting advances in our understanding of animal demographics, evolutionary trajectories and management systems. Prospects to develop new tools and approaches for tracking predation while designing studies to more effectively limit bias are an important frontier for understanding ecosystems and addressing human–wildlife conflicts. Given great uncertainties about environmen-tal change and intensifying conflicts between humans and predators, effective study designs integrating electronic tagging to study predation have a promising future in fundamental and applied ecologypublishedVersio