ORCID Profile
0000-0003-3807-7524
Current Organisations
University of Agder
,
Institute of Marine Research
,
University of Oslo
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Publisher: Wiley
Date: 23-03-2020
DOI: 10.1002/EAP.2108
Publisher: Public Library of Science (PLoS)
Date: 06-09-2013
Publisher: Canadian Science Publishing
Date: 2017
Abstract: Knowledge on mortality causes is key for an effective management of animal populations and can help to restore depleted fish stocks. Here we investigated the mortality dynamics of coastal Atlantic cod (Gadus morhua) in Skagerrak, southern Norway, by analyzing local mark–recapture and recovery data collected from 2005 to 2013 (N = 9360 fish, mean length = 41 cm, range = 16–93 cm). By applying multi-event models to the data, we could link field observations to multiple “dead states” and estimate the proportion of deaths associated with different fishing gears while controlling for unobserved mortality and detection errors. Deaths due to hand lines and fixed gear types were dominant compared with other causes, especially in legal-sized cod (≥40 cm). Gear-specific mortality changed over time and between size classes, but annual survival remained low and stable (∼0.3). Assuming fully additive mortality, we predicted annual survival of cod to be above 0.5 if only one or both of the dominant gear types were removed, providing insights on the relative impact of erse harvesting practices on local population dynamics.
Publisher: Wiley
Date: 09-10-2022
DOI: 10.1111/EVA.13483
Abstract: The effects of marine reserves on the life history and demography of the protected populations are well‐established, typically increasing population density and body size. However, little is known about how marine reserves may alter the behavior of the populations that are the target of protection. In theory, marine reserves can relax selection on spatial behavioral phenotypes that were previously targeted by the fishery and also drive selection in favor of less mobile in iduals. In this study, we used acoustic telemetry to monitor the in idual spatial behavior of 566 Atlantic cod ( Gadus morhua Linnaeus, 1758) moving within a marine reserve and a control site in southern Norway, starting 1 year before the implementation of the marine reserve and lasting up to 9 years after. Following a before‐after‐control‐impact approach, we investigated changes in (1) survival, (2) selection acting on behavioral traits, and (3) mean behavioral phenotypes, after the implementation of the marine reserve. We focused on three behavioral traits commonly used to describe the mobility of aquatic animals: home range size, depth position, and diel vertical migration range. Survival increased after reserve implementation, but contrary to our expectations, it subsequently decreased to preprotection levels after just 3 years. Further, we found no significance in selection patterns acting on any of the three behavioral traits after reserve implementation. Although some changes related to water column use (the tendency to occupy deeper waters) were observed in the marine reserve after 9 years, they cannot unequivocally be attributed to protection. Our results show that survival and behavioral responses to marine reserves in some cases may be more complex than previously anticipated and highlight the need for appropriately scaled management experiments and more integrated approaches to understand the effects of marine protected areas on harvested aquatic species.
Publisher: Canadian Science Publishing
Date: 02-2015
Abstract: Studying the sublethal effects of catch-and-release (C& R) is challenging, as there are several potential sources of bias. For ex le, if behavioural alterations immediately after the release event are to be studied, separation of tagging effects from actual C& R effects is required, which is a challenge in the wild, particularly in marine environments. To investigate the effects of C& R on Atlantic cod (Gadus morhua) in their natural environment, 80 cod were caught in fyke nets, fitted with acoustic transmitters, and released. After recovery from tagging and handling for at least 14 days, nine in iduals were recaptured and released at least once during experimental angling, following best release practice. All cod survived the C& R event and did not show any large-scale behavioural changes (i.e., changes in diel vertical migrations). However, analysis of small-scale vertical movements showed that three in iduals underwent short-term alterations (e.g., reduced or increased swimming activity). This study showed that pretagging fish with acoustic transmitters before experimental angling is an option when investigating fish behaviour immediately after the release event in marine environments. Moreover, release guidelines for cod should be developed, as cod can recover quickly if caught in shallow waters ( m) and properly handled and released.
Publisher: Springer Science and Business Media LLC
Date: 17-09-2011
Publisher: Inter-Research Science Center
Date: 11-02-2010
DOI: 10.3354/MEPS08397
Publisher: Wiley
Date: 26-07-2020
Publisher: Inter-Research Science Center
Date: 16-05-2011
DOI: 10.3354/MEPS09102
Publisher: Elsevier BV
Date: 05-2021
Publisher: Canadian Science Publishing
Date: 03-2012
DOI: 10.1139/F2011-179
Abstract: Tracking data have previously been used to define animal movement patterns through two-dimensional (2D) kernel utilization distributions and separate analysis of vertical locations. Here we describe the use of three-dimensional (3D) kernel utilization distributions to estimate the volumetric space use of in iduals based on tracking data and to estimate the overlap in activity space between in iduals. Data from European eels ( Anguilla anguilla ) from Norwegian coastal waters were used to compare the information conveyed by 2D and 3D activity space estimates and the utility of this approach for aquatic species. The use of 3D kernels produced detailed representations of space use in A. anguilla that permitted examination of depth use in a geographic context. Comparison of 2D and 3D home ranges showed that 2D analyses overestimated the amount of overlap between in iduals by 13%–20%, because in iduals sometimes occurred in the same location but used different depths. Hence, the 3D approach provided more comprehensive representations of animal movement in three dimensions while producing a metric that can be used for testing hypotheses relating to scientific descriptions of activity space, habitat use, and movement parameters.
Publisher: Elsevier BV
Date: 10-2022
Publisher: Wiley
Date: 03-07-2015
Publisher: Public Library of Science (PLoS)
Date: 05-11-2014
Publisher: Inter-Research Science Center
Date: 02-10-2013
DOI: 10.3354/MEPS10459
Publisher: Elsevier BV
Date: 11-2021
Publisher: Wiley
Date: 14-05-2021
Abstract: Understanding the responses of aquatic animals to temperature variability is essential to predict impacts of future climate change and to inform conservation and management. Most ectotherms such as fish are expected to adjust their behaviour to avoid extreme temperatures and minimize acute changes in body temperature. In coastal Skagerrak, Norway, sea surface temperature (SST) ranges seasonally from 0 to over 20°C, representing a challenge to the fish community which includes cold‐, cool‐ and warm‐water affinity species. By acoustically tracking 111 in iduals of Atlantic cod Gadus morhua , pollack Pollachius pollachius and ballan wrasse Labrus bergylta in 2015–2018, we examined how coexisting species within a fish community adjusted their behaviour (i.e. vertical distribution in the water column and habitat selection) to cope with the thermal variation. Mixed‐effect models showed that thermal preference was a main driver of behaviour and habitat use of the fish community in a southern Norwegian fjord. Cod used colder waters, compared with pollack and ballan wrasse. Increases in SST during summer were associated with the use of deeper, colder waters by cod, especially by larger in iduals, and conversely with the occupancy of shallower areas by pollack and ballan wrasse. During winter, when SST dropped and the thermal stratification reversed, pollack and ballan wrasse moved to deeper, relatively warmer areas, while cod selected shallower, colder habitats. Although habitat selection was affected by temperature, species‐specific habitat selection was observed even when temperature was similar throughout habitats. This study shows how cohabiting fish species respond to thermal heterogeneity, suggesting that (a) temperature regulates the access to the different depths and habitats and (b) behavioural plasticity may be an important factor for coping with temperature variability and potentially for adaptation to climate change.
Publisher: Canadian Science Publishing
Date: 07-2011
DOI: 10.1139/F2011-053
Abstract: Marine reserves are perceived as a critical component in successful rebuilding of overharvested marine populations. Such efforts can be particularly successful in species with limited movement rates. However, long-term data on movement is sparse for most marine species. Here, we investigated space use in European lobster ( Homarus gammarus ) by ultrasonic tracking in a reserve on the Norwegian Skagerrak coast from September 2006 to August 2007. Over the period, 95% of tagged lobsters remained either within the reserve or near reserve boundaries. Home range estimates based on a kernel density estimator of the 95% utilization distribution ranged from 5728 to 41 548 m 2 (mean 19 879 ± 2152 m 2 standard error), representing 0.57%–4.15% of the reserve area (1 km 2 ), with no significant difference among males, non-ovigerous females, and ovigerous females for an overlapping observation period of 242 days. Logistic regression predicted average time to reach 50% and 95% of minimum convex polygon home range area at 98 and 259 days, respectively. These results show that European lobsters can be resident with limited home ranges. Small coastal reserves can be designed to afford complete or partial protection by letting boundaries engulf or intersect patches of habitat preferred by this species.
Publisher: Wiley
Date: 26-07-2021
DOI: 10.1002/ECE3.7939
Abstract: Coexistence of ecotypes, genetically ergent population units, is a widespread phenomenon, potentially affecting ecosystem functioning and local food web stability. In coastal Skagerrak, Atlantic cod ( Gadus morhua ) occur as two such coexisting ecotypes. We applied a combination of acoustic telemetry, genotyping, and stable isotope analysis to 72 in iduals to investigate movement ecology and food niche of putative local “Fjord” and putative oceanic “North Sea” ecotypes—thus named based on previous molecular studies. Genotyping and in idual origin assignment suggested 41 in iduals were Fjord and 31 were North Sea ecotypes. Both ecotypes were found throughout the fjord. Seven percent of Fjord ecotype in iduals left the study system during the study while 42% of North Sea in iduals left, potentially homing to natal spawning grounds. Home range sizes were similar for the two ecotypes but highly variable among in iduals. Fjord ecotype cod had significantly higher δ 13 C and δ 15 N stable isotope values than North Sea ecotype cod, suggesting they exploited different food niches. The results suggest coexisting ecotypes may possess innate differences in feeding and movement ecologies and may thus fill different functional roles in marine ecosystems. This highlights the importance of conserving interconnected populations to ensure stable ecosystem functioning and food web structures.
Publisher: Inter-Research Science Center
Date: 03-02-2015
DOI: 10.3354/MEPS11103
Publisher: Inter-Research Science Center
Date: 07-10-2021
DOI: 10.3354/AEI00415
Abstract: Salmon farming in marine net pens is a major activity in many temperate regions. This industry may affect coastal ecosystems in several ways, such as with waste pollution and parasite spillover. Less is known about the extent to which salmon farming disrupts the use of inshore spawning grounds by wild fish, such as the Atlantic cod Gadus morhua . Acoustic telemetry was therefore used to explore cod space use during the spawning season in a coastal region in mid-Norway with multiple salmon farms. Acoustic receivers were placed in clusters at 5 known cod spawning grounds and 6 nearby salmon farms. Data from 481 adult cod caught at the spawning grounds during 2017-2019 and equipped with acoustic telemetry transmitters were analysed. Overall, fewer fish were detected at farms than spawning grounds, even when accounting for distance from release point. In idual cod residency (days detected / duration of spawning period) was generally higher at the spawning grounds close to farms but low at the farms themselves, with little apparent spawning at the farm localities. In contrast, spawning was clearly occurring at the nearby spawning grounds, with cod spending weeks (n = 316) or months (n = 158) there during the spawning period. Males had longer residence times at spawning grounds than females, likely linked to the cod mating system. Overall, we found little support for the assertion that salmon farms disrupt inshore spawning dynamics of cod using nearby spawning grounds presently, either by attracting spawners to farms or causing fish to leave these grounds.
Publisher: Wiley
Date: 17-04-2015
DOI: 10.1002/ECE3.1496
Publisher: Wiley
Date: 05-2021
DOI: 10.1111/EVA.13242
Abstract: Marine reserves can protect fish populations by increasing abundance and body size, but less is known about the effect of protection on fish behaviour. We looked for in idual consistency in movement behaviours of sea trout in the marine habitat using acoustic telemetry to investigate whether they represent personality traits and if so, do they affect survival in relation to protection offered by a marine reserve. Home range size had a repeatability of 0.21, suggesting that it represents a personality trait, while mean swimming depth, activity and diurnal vertical migration were not repeatable movement behaviours. The effect of home range size on survival differed depending on the proportion of time fish spent in the reserve, where in iduals spending more time in the reserve experienced a decrease in survival with larger home ranges while in iduals spending little time in the reserve experienced an increase in survival with larger home ranges. We suggest that the ersity of fish home range sizes could be preserved by establishing networks of marine reserves encompassing different habitat types, ensuring both a heterogeneity in environmental conditions and fishing pressure.
Publisher: Wiley
Date: 22-05-2020
DOI: 10.1111/EVA.12992
Publisher: Wiley
Date: 18-12-2018
DOI: 10.1002/ECE3.4760
Publisher: Elsevier BV
Date: 02-2017
Publisher: The Royal Society
Date: 07-03-2013
Abstract: Marine protected areas (MPAs) are increasingly implemented as tools to conserve and manage fisheries and target species. Because there are opportunity costs to conservation, there is a need for science-based assessment of MPAs. Here, we present one of the northernmost documentations of MPA effects to date, demonstrated by a replicated before–after control-impact (BACI) approach. In 2006, MPAs were implemented along the Norwegian Skagerrak coast offering complete protection to shellfish and partial protection to fish. By 2010, European lobster ( Homarus gammarus ) catch-per-unit-effort (CPUE) had increased by 245 per cent in MPAs, whereas CPUE in control areas had increased by 87 per cent. Mean size of lobsters increased by 13 per cent in MPAs, whereas increase in control areas was negligible. Furthermore, MPA-responses and population development in control areas varied significantly among regions. This illustrates the importance of a replicated BACI design for reaching robust conclusions and management decisions. Partial protection of Atlantic cod ( Gadus morhua ) was followed by an increase in population density and body size compared with control areas. By 2010, MPA cod were on average 5 cm longer than in any of the control areas. MPAs can be useful management tools in rebuilding and conserving portions of depleted lobster populations in northern temperate waters, and even for a mobile temperate fish species such as the Atlantic cod.
Publisher: Wiley
Date: 22-03-2018
DOI: 10.1111/EVA.12611
Publisher: Wiley
Date: 16-07-2015
DOI: 10.1111/JFB.12731
Abstract: Acoustic tags and receivers were used to investigate the spatial ecology of coastal Atlantic cod Gadus morhua (n = 32, mean fork length: 50 cm, range: 33-80 cm) on the Norwegian Skagerrak coast in 2012. Monthly home ranges (HR), swimming activity and depth use varied considerably among in iduals and through the months of June, July and August. HR sizes for the period ranged from 0.25 to 5.20 km2 (mean = 2.30 km2. Two thirds of the tagged G. morhua were infected with black spot disease Cryptocotyle lingua parasites these fish had larger HRs and occupied deeper water compared with non-infected fish. The infected fish also tended to be more active in terms of horizontal swimming. From an ecological and evolutionary perspective, any environmental change that modifies G. morhua behaviour may therefore also alter the parasite load of the population, and its conservation and fishery status.
Publisher: Public Library of Science (PLoS)
Date: 17-02-2012
Publisher: Wiley
Date: 12-06-2012
DOI: 10.1002/ECE3.244
Publisher: Elsevier BV
Date: 12-2012
Publisher: Inter-Research Science Center
Date: 14-05-2018
DOI: 10.3354/MEPS12546
Publisher: Public Library of Science (PLoS)
Date: 09-03-2016
Publisher: The Royal Society
Date: 16-01-2019
Abstract: Marine protected areas (MPAs) are considered viable fisheries management tools due to their potential benefits of adult spillover and recruitment subsidy to nearby fisheries. However, before–after control–impact studies that explore the biological and fishery effects of MPAs to surrounding fisheries are scarce. We present results from a fine-scale spatial gradient study conducted before and after the implementation of a 5 km 2 lobster MPA in southern Norway. A significant nonlinear response in lobster abundance, estimated as catch-per-unit-effort (CPUE) from experimental fishing, was detected within 2 years of protection. After 4 years, CPUE values inside the MPA had increased by a magnitude of 2.6 compared to before-protection values. CPUE showed a significant nonlinear decline from the centre of the MPA, with a depression immediately outside the border and a plateau in fished areas. Overall fishing pressure almost doubled over the course of the study. The highest increase in fishing pressure (by a magnitude of 3) was recorded within 1 km of the MPA border, providing a plausible cause for the depression in CPUE. Taken together, these results demonstrate the need to regulate fishing pressure in surrounding areas when MPAs are implemented as fishery management tools.
Publisher: Wiley
Date: 03-2019
DOI: 10.1111/MEC.15010
Publisher: Inter-Research Science Center
Date: 27-10-2022
DOI: 10.3354/AEI00440
Abstract: Wild fish aggregate at aquaculture net-pens, but the underlying mechanisms are not fully understood. This study examined how salmon farms attract coastal Atlantic cod Gadus morhua from their inshore spawning grounds. Acoustic receivers were deployed at 5 known cod spawning grounds and 6 salmon Salmo salar farms located at varying distances from these grounds in a mid-Norway study site. Cod were caught at each spawning ground annually from 2017-2019, fitted with acoustic transmitters and released (n = 535). A total of 289 tagged cod (54%) were detected at the salmon farms, with more cod detected at farms closest to the focal spawning grounds and at operational farms. The latter result is likely linked to the availability of feeding opportunities at farm locations. Those cod that were detected by the receivers spent less time at farms farther from their release locations. For the farm-associated cod, 70% were detected for wk at the farms. However, 48 cod spent mo close to the farms, with 1 in idual staying 720 d underneath the farm. A total of 135 cod visited 2 or more farms, with farms in proximity more connected in terms of inter-farm movement. Some of the cod utilizing these local spawning grounds likely have considerable dietary input from salmon feed.
Publisher: Wiley
Date: 12-06-2009
Publisher: Wiley
Date: 2011
Publisher: Elsevier BV
Date: 11-2012
Publisher: Wiley
Date: 05-07-2018
Publisher: Canadian Science Publishing
Date: 10-2013
Abstract: Fishing can have profound impacts on the ecology and evolution of marine populations. Understanding population-level changes ultimately depends on knowledge about in idual survival and how it varies in time and space. We used acoustic tags and a network of receivers to monitor in idual behaviour and fate of European lobster (Homarus gammarus) exposed to commercial and recreational trap fisheries on the Norwegian Skagerrak coast. In August 2011, 50 male lobsters above minimum legal size were tagged and monitored before and during the lobster fishing season. We also quantified the spatial and temporal variation in fishing activity. There was no significant effect of home-range size on the probability of surviving the fishery. However, there was substantial fine-scale spatial variation in fishing activity, and lobsters with short-term home ranges positioned away from trap-dense areas had a significantly higher survival probability. Also, the overall survival probability of 16.7% suggests that fishing depletes the catchable lobster population at a high rate. The current harvesting regime may drive selection in favour of movement behaviours avoiding habitats typically targeted by fishers.
Publisher: Wiley
Date: 20-05-2019
DOI: 10.1002/ECE3.5224
Publisher: Wiley
Date: 20-09-2016
DOI: 10.1111/FAF.12188
Publisher: Wiley
Date: 11-04-2022
DOI: 10.1111/FAF.12661
Publisher: Wiley
Date: 16-10-2023
Publisher: Wiley
Date: 23-11-2015
Abstract: Habitat selection is a complex process, which involves behavioural decisions guided by the multiple needs and constraints faced by in iduals. Climate-induced changes in environmental conditions may alter those trade-offs and resulting habitat use patterns. In this study, we investigated the effect of sea temperature on habitat selection and habitat use of acoustically tagged Atlantic cod (Gadus morhua) at the Norwegian Skagerrak coast. Significant relationships between ocean temperature and habitat selection and use were found. Under favourable sea temperature thresholds (<16 °C), cod selected vegetated habitats, such as eelgrass and macroalgae beds, available in shallow areas. Selection for those habitats was especially high at night, when cod tended to ascend to shallower areas, presumably to feed. Selection and use of those habitats decreased significantly as temperature rose. Under increased sea surface temperature conditions, cod were absent from vegetated shallow habitats, both during the day and night, and selected instead non-vegetated rocky bottoms and sand habitats, available in deeper, colder areas. This study shows the dynamic nature of habitat selection and strongly suggests that cod in this region have to trade off food availability against favourable temperature conditions. Future increases in ocean temperature are expected to further influence the spatial behaviour of marine fish, potentially affecting in idual fitness and population dynamics.
Location: Austria
No related grants have been discovered for Esben Moland Olsen.