J. Rasmus Nielsen

Vertical migration and dispersion of sprat (Sprattus sprattus) and herring (Clupea harengus) schools at dusk in the Baltic Sea

In populations of herring (Clupea harengus) or sprat (Sprattus sprattus), one typically observes a pattern of schools forming at dawn and dispersing at dusk, usually combined with vertical migration. This behaviour influences interactions with other species; hence a better understanding of the processes could contribute to deeper insight into ecosystem dynamics. This paper reports field measurements of the dispersal at dusk and examines two hypotheses through statistical modelling: that the vertical migration and the dissolution of schools is determined by decrease in light intensity, and that the dissolution of schools can be modelled by diffusion, i.e. active repulsion is not required. The field measurements were obtained during 3 days in March at one location in the Baltic Sea and included continuous hydroacoustical monitoring, trawl samples, and hydrographical CTD data. Echogram patterns were analysed using the school detection module in Echoview ® and local light intensities were calculated using a model for surface illuminance. The data and the analysis support that schools migrate upwards during dusk, possibly trying to remain aggregated by keeping the local light intensities above a critical threshold, that schools initiate their dissolution when ambient light intensity drops below this critical threshold, and that fish subsequently swim in an uncorrelated random walk pattern.

The footprint of bottom trawling in European waters: distribution, intensity, and seabed integrity

Mapping trawling pressure on the benthic habitats is needed as background to support an ecosystem approach to fisheries management (EAFM). The extent and intensity of bottom trawling on the European continental shelf (0–1000 m) was analyzed from logbook statistics and VMS data for 2010, 2011 and 2012 at a resolution of 1×1 minutes longitude and latitude. Trawling intensity profiles with seabed impact at the surface and subsurface level are presented for 14 management areas in the North-east Atlantic, Baltic Sea and Mediterranean Sea. The footprint (proportion of the seabed trawled 1 or more times every ten years) ranged between 40–90% across EUNIS habitats with largest footprints observed in sandy (A5.2) and muddy (A5.3) habitats. The footprint of the management areas ranged between 52-99% and 5-94% for the depth zone from 0–200 m (Shallow) and from 201–1000 m (Deep), respectively. The footprint was estimated as the total area of all grid cells that were trawled fully or partially. Excluding these untrawled proportions reduced the footprint estimates to 28-85% and 2-77%. Mean trawling intensity ranged between 0.5 and almost 8.5 times per year, but was less in the Deep zone with a maximum intensity of 6.4 times per year. Highest intensities were recorded in the Skagerrak–Kattegat and Adriatic Sea. Largest footprints per unit landings were observed in the Mediterranean Sea. Bottom trawling was highly aggregated. The seabed area where 90% of the effort occurred comprised between 11% and 65% (median 44%) of the total area trawled. Using the longevity distribution of the untrawled infaunal community, the seabed integrity was estimated as the proportion of the biomass of benthic taxa where the trawling interval at the subsurface level exceeds their life span. Seabed integrity was low (<0.1) in large parts of the European continental shelfs, although smaller pockets of seabed with higher integrity values occur. The methods developed here integrate official fishing effort statistics and industry-based gear information to provide high-resolution pressure maps and indicators, which greatly improve the basis for assessing and managing benthic pressure from bottom trawling. Further they provide quantitative estimates of trawling impact on a continuous scale by which managers can steer.

Distribution, density and abundance of the western Baltic herring (Clupea harengus) in the Sound (ICES Subdivision 23) in relation to hydrographical features

Abstract Biomass and duration of the over-wintering period of the Rugen spring spawning herring stock (RHS) in the Sound (ICES Subdivison 23) were investigated as well as possible hydrographical factors affecting relative distribution and triggering southwards migration towards the spawning grounds. Monitoring was performed during 27 surveys over a 6-year period (1993–1998). Abundance of 45–165xa0000xa0t in August–February, 5–60xa0000xa0t in March–May, and

A Statistical Model for Estimation of Fish Density Including Correlation in Size, Space, Time and between Species from Research Survey Data

Trawl survey data with high spatial and seasonal coverage were analysed using a variant of the Log Gaussian Cox Process (LGCP) statistical model to estimate unbiased relative fish densities. The model estimates correlations between observations according to time, space, and fish size and includes zero observations and over-dispersion. The model utilises the fact the correlation between numbers of fish caught increases when the distance in space and time between the fish decreases, and the correlation between size groups in a haul increases when the difference in size decreases. Here the model is extended in two ways. Instead of assuming a natural scale size correlation, the model is further developed to allow for a transformed length scale. Furthermore, in the present application, the spatial- and size-dependent correlation between species was included. For cod (Gadus morhua) and whiting (Merlangius merlangus), a common structured size correlation was fitted, and a separable structure between the time and space-size correlation was found for each species, whereas more complex structures were required to describe the correlation between species (and space-size). The within-species time correlation is strong, whereas the correlations between the species are weaker over time but strong within the year.

Bio-economic evaluation of implementing trawl fishing gear with different selectivity

The paper develops a biological-economic evaluation tool to analyse the consequences for trawl fishers of implementing more selective fishing technologies. This is done by merging a dynamic biological population model and an economic cost-benefit evaluation framework to describe the consequences for the fish stocks, fishermen and society. The bio-economic evaluation is applied to the case of the Danish trawl fishery in Kattegat and Skagerrak, which experiences a high level of discards and bycatches of several species. Four different kinds of selectivity scenarios are evaluated in comparison with a baseline. The results from the evaluation are indicators for the consequences on ecological and economic levels. The results show that implementation of different selective fishing gear in the Kattegat and Skagerrak mixed trawl fisheries generally implies a trade off over time between rebuilding the stocks and economic loss. Moreover, the analysis shows that implementation of more selective gear is not always beneficial.

Localisation of nursery areas based on comparative analyses of the horizontal and vertical distribution patterns of juvenile Baltic cod (Gadus morhua).

Knowledge of the spatial distribution of juvenile cod is essential for obtaining precise recruitment data to conduct sustainable management of the eastern and western Baltic cod stocks. In this study, the horizontal and vertical distribution and density patterns of settled juvenile 0- and 1-group Baltic cod are determined, and their nursery areas are localised according to the environmental factors affecting them. Comparative statistical analyses of biological, hydrographic and hydroacoustic data are carried out based on standard ICES demersal trawl surveys and special integrated trawl and acoustic research surveys. Horizontal distribution maps for the 2001–2010 cohorts of juvenile cod are further generated by applying a statistical log-Gaussian Cox process model to the standard trawl survey data. The analyses indicate size-dependent horizontal and distinct vertical and diurnal distribution patterns related to the seabed topography, water layer depth, and the presence of hydrographic frontal zones (pycnoclines) as well as intraspecific patterns in relation to the presence of adult cod. The extent of the nursery areas also depends on the cod year class strength. Juvenile cod (≥3 cm) are present in all areas of the central Baltic Sea (CBS), showing broad dispersal. However, their highest density in the Baltic Basins is found at localities with a 40–70 m bottom depth in waters with oxygen concentrations above 2 ml O2.l−1 and temperatures above 5°C. The smallest juveniles are also found in deep sea localities down to a 100 m depth and at oxygen concentrations between 2–4 ml O2.l−1. The vertical, diurnally stratified and repeated trawling and hydroacoustic target strength-depth distributions obtained from the special surveys show juvenile cod concentrations in frontal zone water layers (pycnocline). However, the analyses indicate that in the CBS, juvenile cod of all sizes do not appear to aggregate in dense schooling patterns, which differs from what has been reported from the North Sea.

Effects of changes in stock productivity and mixing on sustainable fishing and economic viability

viability DTU Orbit (07/11/2019) Effects of changes in stock productivity and mixing on sustainable fishing and economic viability Within the new FMSY European paradigm, this paper shows how a combination of changes in fish stock mixing, nonstationarity in productivity, and constraints on unit stock concepts undermine the effective management of fisheries, especially when management reference points are not adjusted accordingly. Recent changes in stock structures, conditions and stock mixing between eastern and western Baltic cod can jeopardize the reliability of stock assessments and of the fishery economy. We modelled how different management, individual vessel decision-making, and stock growth and mixing scenarios have induced alternative individual vessel spatial effort allocation and economic performance by affecting fishing costs and by changing the relative stock abundance and size distribution. Stock mixing heavily influences profit and stock abundance for stocks that have experienced increased fishing mortality (F) levels. Western cod F has increased from a higher total allowed catches (TAC) advised in the medium-term due to the westward migration of eastern cod while eastern cod F has increased from reduced growth in the east. Greater pressures on western cod and decreased eastern cod growth and conditions greatly reduce the overall cod spawning stock biomass, thus changing the landing size composition and associated fishery profits. As a cumulative effect, fishing efforts are redirected towards western areas depending on management (quotas). However, total profits are less affected when traditional fishing opportunities and switching possibilities for other species and areas are maintained. Our evaluation indicates that current management mechanisms cannot correct for potential detrimental effects on cod fisheries when effort re-allocation changes landing origins. By investigating different economic starting conditions we further show that Baltic cod mis-management could have resulted in unintended unequal (skewed) impacts and serious consequences for certain fleets and fishing communities compared with others. Our management strategy evaluation is instrumental in capturing non-linear effects of different recommendations on sustainability and economic viability, and we show that fixed F-values management is likely not an attainable or sufficient goal in ensuring the sustainability and viability of fisheries and stocks given changing biological conditions

Chapter 7 Effort and capacity-based fisheries management

Publisher Summary Effort based fisheries management is an alternative management system to the total allowable catch (TAC) system. The European Union (EU) has started to use both management systems in parallel by supplementing the traditional TAC fleet capacity restrictions with sea-day restrictions from the year 2003. However, global examples of effort-based management systems are scarce. This chapter describes and evaluates the knowledge base of command and control effort and capacity regulation. In this chapter the knowledge base of management and advice for implementing effort regulation, in partly national fisheries systems and partly multi-jurisdictional systems with the relative stability for historical rights of international resource allocation and sharing are evaluated. Knowledge about fishers behavior in relation to regulations is also addressed, and economic incentives for effort allocation, switching among fisheries, discarding, and fleet adaptation are discussed. Overall knowledge of compliance and incentives to comply with regulations and the concept of legitimacy are comparatively addressed for effort management and TAC-based management among the selected examples.

New policies may call for new approaches: the case of the Swedish Norway lobster (Nephrops norvegicus) fisheries in the Kattegat and Skagerrak

&NA; The European Common Fisheries Policy has in its 2013 reform increased in complexity, such as a call for coherence with the Marine Strategy Framework Directive and a landing obligation, posing new requirements and challenges to managers, scientists and the fishing industry. Therefore, re‐evaluations of current practice are important as a basis for management actions. The Swedish fishery for Norway lobster (Nephrops norvegicus) in the Kattegat‐Skagerrak area provides an interesting case study of relevance to emerging policies. Sprung from an unbalance in available fish‐ and Nephrops quotas and an ambition to protect coastal areas, the current fishery has been directed towards three separate fisheries (mixed trawling, directed trawling using a sorting grid and creeling). Studying direct and indirect effects from alternative Swedish quota allocations among gear types is therefore interesting. Accordingly, a screening study was conducted, taking into consideration area‐gear interactions in catch rates, to compare the three different fisheries regarding quantified pressures on the target species, the by‐catch species, and on the seafloor, as well as to qualitatively discuss social and economic dimensions. In the next step, alternative quota allocations were studied. In Swedish fisheries, we show that creeling offers a substantial reduction of fishing mortality of both undersized Nephrops and fish and a reduced seafloor pressure per landed kilo of Nephrops. Given that the fishing areas in many cases may be interchangeable between gears, allocating a larger quota share to creels in the Swedish fishery would therefore contribute to the integration of fisheries‐ and environmental management as called for in the new policies.

Chapter 10 Ecological side-effects of fishing from the fisheries management perspective

Publisher Summary The effects of fishing on the marine environment can be broadly categorized into direct and indirect ones. Direct effects include: (1) mortality of target and by-catch species; (2) changes in food availability and competition for resources among certain predators, such as marine mammals, large fish, and sea birds; (3) discards of unwanted catch and fish offal, and (4) physical and biological disturbance of the seabed (benthic habitats and benthic species). Indirect effects include: (1) changes in the genetic composition of fished populations and communities; (2) changes in species and size composition of the marine communities; and (3) changes in energy flow in the ecosystem. Environmental impacts of fishing can be either short-term, or long-term with more irreversible effects, the first ones of which being more easily evaluated. Because of such complexity that exists in marine ecosystems, less holistic approaches to formulating specific management elements and objectives, in relation to small-scale and local ecosystem side-effects of fisheries, are ongoing in relation to OSPAR and the Bergen Declaration. This chapter summarizes the main ecological side-effects of fishing and discusses various management approaches and actions for facing these effects.