Assessing the potential for introducing resident brackish-water pike ( Esox lucius ) to restore depleted Baltic Sea populations

In the Baltic Sea, the large predatory fish northern pike (Esox lucius L.) is important for both recreational fisheries and ecosystem functioning. As existing fishery-independent surveys do not adequately monitor pike populations, a general lack of knowledge on population status and trends poses challenges for management. Here we use recreational angling data as an alternative method to describe pike population development along the Swedish Baltic Sea coast from 1938 onwards and assess the change over time in potential mortality factors by estimating harvest by fisheries and consumption by large predators. Data from a Swedish national register on trophy-sized pike (>12 kg) showed that numbers and maximum sizes peaked in 1990–1995, after which declines in both metrics are evident. In logbooks from a sport fishing club in the Stockholm archipelago, a simultaneous decrease in maximum sizes of pike was observed, together with a decrease in the total number of pike caught per fishing day. Jointly, these data indicate a decline in the abundance of large pike since around 1990. While commercial pike fisheries in the Baltic Sea have decreased continuously since the 1950s, recreational fishing increased after 1985, when Swedish coastal waters were made open access to anglers. The declines in large pike starting in the 1990s could, thus, have been driven by an increase in mortality from recreational fisheries, which at the time primarily practiced catch and kill. Since the 2000s, bag and slot length limits, spawning closures, and a general increase in catch-and-release fishing has reduced the landings of pike in recreational fisheries. Despite these fisheries regulations and higher release rates the decline in catches of large pike has continued, indicating an effect of other mortality factors. The strong growth of grey seal (Halichoerus grypus) and great cormorant (Phalacrocorax carbo sinensis) populations suggest that predation pressure on pike has increased over time. In the Stockholm archipelago these two predators were estimated to remove 5–18 times (range based on different diet composition estimates) more pike biomass than total fisheries landings in 2014–2017. To improve the situation for northern pike in the Baltic Sea managers need to consider both fisheries restrictions and measures to decrease predation pressure and increase recruitment. Catch data from recreational fisheries may be used to evaluate such management efforts by providing information on the population development of this keystone species.

Genetic structure of pike (Esox lucius Linnaeus, 1758) populations along the Polish coast of the southern Baltic Sea: Comparison to Danish brackish population

The northern pike (Esox lucius) is an iconic predatory fish species of significant recreational value and ecological role in the Baltic Sea. Some earlier studies indicate local declines of pike in the region, but a thorough spatial evaluation of regional population trends of pike in the Baltic Sea is lacking. In this study, we collate data from 59 unique time-series from fisheries landings and fishery-independent monitoring programs to address temporal trends in pike populations since the mid-2000′s in eight countries surrounding the Baltic Sea. In a common analysis considering all time-series in concert, we found indications of an overall regional temporal decline of pike in the Baltic Sea, but trends differed among countries. Individual negative trends in time-series were moreover found in several regions of the Baltic Sea, but predominantly so in the central and southern parts, while positive trends were only found in Estonia and northern Finland. The mix of data used in this study is inherently noisy and to some extent of uncertain quality, but as a result of the overall negative trends, together with the socioeconomic and ecological importance of pike in coastal areas of the Baltic Sea, we suggest that actions should be taken to protect and restore pike populations. Management measures should be performed in combination with improved fishery-independent monitoring programs to provide data of better quality and development of citizen-science approaches as a data source for population estimates. Possible measures that could strengthen pike populations include harvest regulations (including size limits, no-take areas and spawning closures), habitat protection and restoration, and an ecosystem-based approach to management considering also the impact of natural predators.

BackgroundPikes, members of genus Esox, are widespread freshwater predators of the northern hemisphere, and important sport fish also. From the Carpathian basin only one species, the northern pike (E. lucius) is noted. At the same time the pike stocks living in this area show high level of phenotypic variance (e.g. various body pattern) and its growth varies highly both among and within populations. These features usually explained by the environmental diversity of the area. Whereas we think that genetic reasons -e.g. the appearance of other/new pike species in the area- may also be responsible for these observed features. Since as no detailed information have been published from the pike populations of this area, so far; we conducted phylogenetic and morphological assay on 88 pike specimens, collected from 49 Middle Danubian sampling sites.ResultsOur phylogenetic surveys showed that the northern pike appear in the study area solely, but all the three of its major lineages (Northern, Circumpolar, Southern) were indicated. Only six specimens represent the Northern lineage, collected from the western part of the study area. The Circumpolar and Southern lineages were common in the Carpathian basin, but the Southern lineage showed higher levels of haplotype diversity than the Circumpolar clade. Which indicates that only the Southern lineage is native in the area, while the other two groups could have appeared in the Middle Danubian system either spontaneously or by human introduction. Moreover, the different clades appeared in the same populations, suggesting the opportunity of inter-lineage hybridisation. From the studied morphologicalal features, the number of scales on the lateral line and the head length showed significant differences among the lineages. At the same time the body pattern of the studied individuals seems to be rather influenced by the ontogenic changes than phylogeny.ConclusionsThe high phenotypic variability of Middle Danubian northern pike populations may be due that all of its three major clades appeared and came in secondary contact in the area. In the within watershed spread of the non-native lineages the human stocking/transfer may play a considerable role.

Objective Coastal populations of Northern Pike Esox lucius in the Baltic Sea are declining in both abundance and individual size. Management measures, including seasonal spawning closures, have been implemented for selected vulnerable populations. To ensure effective management, it is crucial to estimate spawning population densities and relate them to the number of individuals that are caught by recreational angling, as this is a primary factor that influences Northern Pike populations. Methods This study presents findings from a 6-year spatial capture–recapture study using standardized recreational angling in a Baltic Sea archipelago bay. Results The estimated population size of Northern Pike measuring ≥42 cm was 2,449 individuals (95% CI [1,264 to 4,889]), or 65 individuals per hectare, whereas the population of Northern Pike measuring ≥60 cm was estimated at 1,156 individuals (95% CI [647 to 2,079]), or 31 individuals per hectare. On average, two anglers that fished for 4 h captured 1.1% (95% CI [0.6% to 2.1%]) of the total population, equivalent to 27 individuals. Although the maximum captures for a single day amounted to 2.9% (95% CI [1.5% to 5.6%]). These results suggest that if recreational angling is frequent and continuous, a large percentage of the population will be captured, which could likely influence the population density and size structure in the long term. Over the 6-year period, previously captured Northern Pike had a higher recapture probability than those that were not caught previously, indicating a behavioral effect such that some individuals are more susceptible to capture, making a portion of the population more vulnerable. Additionally, there was weak evidence suggesting that smaller Northern Pike (≥42 cm and <60 cm) had a lower capture probability than larger individuals (≥60 cm). However, a bell-shaped length distribution of the captured Northern Pike suggests that smaller individuals may have been undersampled. Grey seals Halichoerus grypus, a piscivorous top predator, were observed in the bay during the fourth year of the study. The presence of seals significantly affected the capture probability of Northern Pike, reducing angling success more than influencing the estimated population density. The presence of seals likely caused a change in the behavior of Northern Pike that resulted in lower catchability rather than reduced population size. Conclusion This study highlights that stock indices based on catch per unit effort in standardized angling can be biased when capture probability varies, such as in the presence of grey seals. However, this bias can be corrected by using capture–recapture models. The insights that are gained from this study can help improve the ongoing monitoring and management of recreational angling and the effects of grey seals, benefiting Northern Pike populations in the Baltic Sea.

Temporal changes and effective population size of an Italian isolated and supportive-breeding managed northern pike ( Esox lucius) population

Eutrophication and reduced grazing have led to an expansion of the common reed (Phragmites australis) in archipelago areas in the Baltic Sea, while at the same time the composition of submerged vegetation has changed. Although reed is important as nursery habitat for many fish species, extensive emergent vegetation may reduce biodiversity and abundance of predatory fish. Pike (Esox lucius) is a predatory fish whose larvae and young-of-the-year find both food and shelter in reed but use different micro-habitats during different life-stages. Here we investigate the influence of reed and submerged vegetation on abundance and body size of adult pike during the spawning season. We predicted that coastal bays with extensive but heterogeneous reed beds with higher cover of submerged vegetation would have more and larger pike than bays with smaller, homogenous reed belts or with less submerged vegetation. To test these predictions, we estimated abundance and size-structure of adult pike from catches in angling fishing among 22 bays in the Stockholm archipelago at the Swedish Baltic Sea coast. Our analyses show that catches of adult pike were positively associated with both extensive reed beds and cover of rooted submerged vegetation. However, pike size was not correlated with any vegetation variable, but instead increased with wave exposure and bay area. Our study suggests that reed beds and submerged vegetation are important for adult pike during the spawning season, and even the most extensive reed beds had no evident negative effect on pike populations. We could not see any clear relationship between emergent reed habitat and cover of submerged vegetation among the studied bays, and conclude that to maintain pike population during the spawning season it is important that coastal bays have sufficient amounts of both reed beds and rooted submerged vegetation.

[This corrects the article DOI: 10.1371/journal.pone.0162277.].

Determining the success of invasive species eradication efforts is challenging because populations at very low abundance are difficult to detect. Environmental DNA (eDNA) sampling has recently emerged as a powerful tool for detecting rare aquatic animals; however, detectable fragments of DNA can persist over time despite absence of the targeted taxa and can therefore complicate eDNA sampling after an eradication event. This complication is a large concern for fish eradication efforts in lakes since killed fish can sink to the bottom and slowly decay. DNA released from these carcasses may remain detectable for long periods. Here, we evaluated the efficacy of eDNA sampling to detect invasive Northern pike (Esox lucius) following piscicide eradication efforts in southcentral Alaskan lakes. We used field observations and experiments to test the sensitivity of our Northern pike eDNA assay and to evaluate the persistence of detectable DNA emitted from Northern pike carcasses. We then used eDNA sampling and traditional sampling (i.e., gillnets) to test for presence of Northern pike in four lakes subjected to a piscicide-treatment designed to eradicate this species. We found that our assay could detect an abundant, free-roaming population of Northern pike and could also detect low-densities of Northern pike held in cages. For these caged Northern pike, probability of detection decreased with distance from the cage. We then stocked three lakes with Northern pike carcasses and collected eDNA samples 7, 35 and 70 days post-stocking. We detected DNA at 7 and 35 days, but not at 70 days. Finally, we collected eDNA samples ~ 230 days after four lakes were subjected to piscicide-treatments and detected Northern pike DNA in 3 of 179 samples, with a single detection at each of three lakes, though we did not catch any Northern pike in gillnets. Taken together, we found that eDNA can help to inform eradication efforts if used in conjunction with multiple lines of inquiry and sampling is delayed long enough to allow full degradation of DNA in the water.

The structure of the habitat is usually crucial for growth and survival of young life stages. Presently, some nursery areas of fish larvae are changing due to eutrophication, e.g. due to enhanced growth of ephemeral filamentous algae at the expense of perennial species. We studied the influence of two habitats, one with filamentous algae (Cladophora glomerata) and the other with bladder wrack (Fucus vesiculosus), on habitat choice of pike larvae (Esox lucius) in the absence/presence of a predator or a competitor. We further tested whether the habitat choice is adaptive in increasing survival under predation threat. In contrast to expectations, pike larvae preferred the habitat with ephemeral filamentous algae to the bladder wrack, thriving in clean waters, independent of the presence/absence of both predator/competitor. In addition, the survival of the larvae was higher in the filamentous algae in the presence of predators, which suggested that the habitat preference of the larvae was adaptive. The structure of the bladderwrack habitat was probably too open for newly hatched larvae, which implies that F. vesiculosus and other large brown algae are not as important refuges for young larvae as previously thought.

Cleithra are thought to accurately record age information and produce the most reliable age estimates relative to other calcified structures (e.g., scales) for long-lived species of Esocidae such as muskellunge Esox masquinongy and northern pike E. lucius. Sagittal otoliths provide the most accurate and precise age estimates for other fish species, yet sagittal otoliths have never been evaluated for age estimation of any species of Esocidae. Our objectives were to determine if: 1) sagittal otoliths provided more precise age estimates than cleithra for northern pike from two populations, and 2) sagittal otolith age estimates differed systematically from cleithrum age estimates for two populations of northern pike. Ages were estimated by three independent individuals with different experience levels from sagittal otoliths and cleithra collected from 66 northern pike (32–101 cm total length) from Devils Lake, North Dakota and 45 northern pike (27–52 cm total length) from Cable Lake, Wisconsin. Cleithrum age estimates were more precise than those from sagittal otoliths for northern pike from Devils Lake, and were similar to sagittal otolith age estimates for northern pike from Cable Lake. Sagittal otolith age estimates were similar to cleithrum age estimates for northern pike from Devils Lake, but were dissimilar for northern pike from Cable Lake. We recommend using cleithra for estimating age of northern pike given that no specialized equipment is required for processing and age estimation. However, other studies are needed to further investigate the use of sagittal otoliths to estimate age of northern pike.

Identification and protection of critical spawning habitat for muskellunge Esox masquinongy and northern pike Esox lucius is important for preserving the reproductive potential of both species. In this study, we implanted miniature radio transmitters through the oviduct into the egg masses of female muskellunge and northern pike just prior to spawning. This non-surgical procedure was a novel approach for identifying spawning sites when transmitters were expelled with the eggs during egg deposition. Preliminary studies in three lakes showed that muskellunge and northern pike deposited many of the transmitters in likely spawning habitat. An inability to find eggs limited our validation of this method, but nevertheless, a relatively high proportion (70%) of northern pike larger than 690 mm (27.2 inches) expelled transmitters in a previously known spawning area in Willow Lake, Minnesota. Shoreline vegetation in that area consisted primarily of sedges Carex spp., and the adjacent water was shallow with substrate consisting of large mats of water bulrush Scirpus subterminalis. A lower proportion (50%) of muskellunge expelled transmitters in Elk Lake, Minnesota. Water depth at likely spawning sites averaged 1.1 m (3.6 feet) and vegetative cover was variable, but Chara spp. was common to most sites. In Moose Lake, Minnesota, containing sympatric populations of muskellunge and northern pike, 60% of muskellunge and 90% of pike expelled transmitters. Chara spp. beds were the predominant substrate where transmitters were expelled in Moose Lake, but the two species deposited transmitters on deepwater bars (3.7–5.2 m) in addition to shallow near-shore habitat. These results suggest more flexibility in depths used for spawning than typically reported for muskellunge and northern pike.

Coastal basins are particularly exposed to the adverse impact of anthropogenic stress. In many places, despite only the seasonal increase in the number of residents, progressive urbanization and associated changes in the catchment characteristics are noticeable. Puck Bay is part of the Gulf of Gdansk and belongs to the Baltic Sea. Although the area of Puck Bay is covered by the Natura 2000 Network, this has not saved it from eutrophication problems. As part of the work on a complex coastal basin analysis (WaterPUCK project), the Soil and Water Assessment Tool (SWAT) model was used to determine the agricultural impact on water quality in rivers with a flow into Puck Bay: Reda, Gizdepka, Płutnica, and Błądzikowski Stream. The results include the loads of nutrients and pesticides that flow out from the agricultural areas of Puck community into Puck Bay. In this article, special attention has been paid to the impact of precipitation on the quality of water at the outflow of rivers into the Bay of Puck, because it is a decisive element in the amount of nutrients leached along with surface runoff to watercourses and then into the Gulf. The distribution of precipitation thus affects the amount of nutrients absorbed by plants. Modeling the effects of agricultural practices, taking into account long-term meteorological forecasts, is helpful in attempts to reduce the amount of pollutants entering the Baltic Sea.

The brackish Baltic Sea is inhabited by several primary freshwater fish species. The pike (Esox lucius) is the largest of these species and acts as the top predator in the marine food web of the Baltic Sea. Prior studies have shown the importance of brackish bays as spawning grounds for the Baltic Sea pike. To identify the spawning habitat of the pike population around the island of Rügen (southern Baltic Sea)—either freshwater streams flowing into the Baltic or brackish lagoons of the Baltic—we used the ICPMS method to quantify the strontium to calcium (Sr:Ca) ratios in sagittal otoliths of 79 adult pike. In total, only six per cent (n = 5) of the analysed Baltic Sea individuals were classified as being of freshwater origin. Ninety‐four per cent (n = 74) of analysed pike were classified as having hatched in brackish water. Compared to previous studies on Baltic Sea pike, our sample shows a high proportion of pike that is of brackish origin: this highlights the adaptive capacity of this species with regard to its ability to reproduce in different salinities. Our data highlight the importance of brackish bays for pike reproduction in the southern Baltic Sea, even if pike live on the upper limit of their salinity tolerance in this area. This is an important finding considering decreases in commercial catches in the last 30 years indicating a population decline supposedly related to recruitment failures.

Denitrification and anammox (anaerobic ammonium oxidation) are the main nitrogen removal pathways. Denitrification is a microbial process in which NO3- is sequentially reduced to dinitrogen gas (N₂) while anammox is the anaerobic microbiological process in which NO2- and NH4+ are converted to N2 under anoxic conditions. Both processes are critical in regulating nitrogen (N) availability in marine ecosystems, particularly in the stratified and oxygen-depleted aquifers such as Baltic Sea. The Baltic Sea, highly complex and semi-enclosed marine ecosystem that contains brackish water due to high freshwater discharge and limited water exchange with the North Sea. The sedimentary nitrogen cycling was studied extensively in the Baltic Sea but still, understanding the nitrogen loss process, especially in the coastal area is challenging. Additionally, studies usually use different methods to assess the N removal rates which disables the comparison of obtained rates and limits the overall understanding of the N cycle. The main aim of the study was to quantify denitrification and anammox rates in surface sediments from various locations in the Baltic Sea. Three coastal sites MP2 (Puck Bay), PB3 (Puck Bay under submarine groundwater discharge (SGD) impact), lagoon MS2 (Szczecin lagoon) and two open-sea sites IDEAL, P1 (Baltic Proper) were selected for this study. Slurry incubation experiments were conducted at each site with the addition of labelled substrates ¹⁵NO₂⁻ and ¹⁵NH₄⁺ to measure denitrification and anammox rates. The addition of ¹⁵NO₂⁻ produced ¹⁴N¹⁴N, ¹⁴N¹⁵N, and ¹⁵N¹⁵N for denitrification, while ¹⁵NH₄⁺ produced ¹⁴N¹⁴N and ¹⁴N¹⁵N for anammox. The denitrification rate in the coastal sites ranged from 1440.82 to 7.21 nM N L-1 d-1, for the open sea sites (IDEAL) 533.42 nM N L-1 d-1 and at P1 consumption of N2 was observed. Apart from MP2, anammox activity was detected at PB3 (32.67 nM N L-1 d-1), MS2 (0.41 nM N L-1 d-1), IDEAL (0.46 nM N L-1 d-1), and P1 (0.67 nM N L-1 d-1). The anammox rates were lower than denitrification at all sites, indicating its minor role in nitrogen removal in the surface sediments of Baltic Sea. However, the presence of anammox highlights the contribution of a diverse microbial community that can increase with the future expansion of anoxic areas in the Baltic Sea. The observed spatial variability in N removal rates is likely influenced by site-specific factors such as organic matter availability, nutrient discharge, and oxygen conditions. However, hypoxic submarine groundwater discharge (SGD), enriched in nutrients and dissolved organic carbon appears to be a key driver of nitrogen removal. Further studies employing similar methodological approaches are essential to better understand nitrogen cycling in marine ecosystems, particularly in coastal areas.AcknowledgmentsThe results were obtained within the framework of the statutory activities of the Polish Academy of Sciences Institute of Oceanology and the research project IDEAL (2019/34/E/ST10/00217) funded by the Polish National Science Centre.