We surveyed Antarctic krill (Euphausia superba) under sea ice using the autonomous underwater vehicle Autosub-2. Krill were concentrated within a band under ice between 1 and 13 kilometers south of the ice edge. Within this band, krill densities were fivefold greater than that of open water. The under-ice environment has long been considered an important habitat for krill, but sampling difficulties have previously prevented direct observations under ice over the scale necessary for robust krill density estimation. Autosub-2 enabled us to make continuous high-resolution measurements of krill density under ice reaching 27 kilometers beyond the ice edge.

Analyses of global fish stocks paint a mixed picture of success, with some holding fishery management responsible for the poor status of many stocks [1-3] or predicting widespread collapse [1, 4]. Some suggest a stable [5] or improving situation [6] in certain jurisdictions. The debate is particularly polarized in the European Union, where the Common Fisheries Policy (CFP) has been criticized for failing to protect stocks [2, 7-10], while others argue that a rebuilding process is underway [11, 12]. We show that substantial change in stock trends occurred in the area around the turn of the century: since then, the fishing pressure (as measured by the exploitation rate) has reduced continuously and there have been increases in biomass, demonstrating the potential for stock recovery. In 2011, for the first time, the majority of assessed stocks, where reference points are defined, were fished sustainably. The reductions in fishing pressure were associated with declines in fishing effort. The last reform of the CFP, in 2002, introduced effort control as part of more enforceable management measures, which were also based on longer-term plans. Further reforms to the CFP are currently being developed, so it is important, when correcting its weaknesses, to also acknowledge and build on the success of a major reduction in the fishing pressure on European fish stocks.

Fernandes, P. G. 2009. Classification trees for species identification of fish-school echotraces. – ICES Journal of Marine Science, 66: 1073–1080. Acoustic surveys provide valuable information on the abundance and distribution of many fish species, but are particularly effective for schooling pelagic fish of commercial importance. However, despite recent advances in multifrequency processing, the technique still requires some subjective judgement when allocating the acoustic data, fish-school echotraces, to particular species—the so-called “scrutiny process”. This is assisted by “ground truth” trawling and operator experience of relating trawl data to echotraces of particular fish schools. In this paper, a method to identify species based on “classification trees” is applied to data from a component of the International North Sea Herring Acoustic Survey. Classification trees may be considered as a variant of decision trees, and have properties that are intuitive to biologists, because they are similar to the keys used for the biological identification of species. The method described here incorporates a multifrequency fish-school filter, image analysis to isolate fish-school echotraces, and finally, a classification-tree system using the multifrequency information from the ground-truthed echotraces that can be translated into a processing tool for objective species allocation. The classification-tree system is compared with the traditional method of expert-based scrutiny. Unlike the latter, however, a measure of uncertainty is attributed to the classification-tree approach and this could be propagated through the acoustic-survey estimation procedure as a component of the total uncertainty in the abundance estimate.

Korneliussen, R. J., Diner, N., Ona, E., Berger, L., and Fernandes, P. G. 2008. Proposals for the collection of multifrequency acoustic data. – ICES Journal of Marine Science, 65: 982–994. Acoustic surveys are used to estimate the abundance and distribution of many fish species, and have been based traditionally on data collected at a single acoustic frequency. Although it has been known for some time that the use of additional frequencies can provide information on the nature of the acoustic target, the knowledge and technology required to combine the so-called “multifrequency data” in an appropriate manner has been limited. The use of several transducers of different frequencies is now common on board research vessels and fishing vessels, so multifrequency data are often collected. In order for these data to be combined appropriately, their physical and spatial characteristics from each frequency should be as similar as possible. We detail the requirements deemed necessary to collect multifrequency data in an appropriate manner. They can be stringent and may not always be achievable, so we also consider the consequences of combining acoustic data originating in transducers with varying degrees of spatial separation and with different beam widths.

Many commercial fish stocks are beginning to recover under more sustainable exploitation regimes. In this study, we document the temporal and spatial changes in one remarkable example of stock recovery: northern European hake (Merluccius merluccius). Analysing data from several scientific surveys, we document a dramatic increase in estimates of biomass between 2004 and 2011 throughout the larger area now occupied by the stock. The largest increase occurred in the North Sea, where hake have been largely absent for over 50 years. Spatio‐temporally resolved commercial landings show that high densities occur in the North Sea only between April and September, suggesting a density‐dependent seasonal habitat expansion to suitable temperature and depth conditions. These changes have implications for the management of the stock which are discussed. Notably, if discards are banned as part of management revisions, the relatively low quota for hake in the North Sea will be a limiting factor (the so‐called ‘choke’ species) which may result in a premature closure of the entire demersal mixed fishery in the North Sea, jeopardizing many commercial fisheries in the region. This example of the unforeseen consequences of improved stewardship highlight the need for a more holistic, regional and responsive approach to managing our marine ecosystems.

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Autonomous underwater vehicles (AUVs) are unmanned submersibles that can be pre-programmed to navigate in three dimensions under water. The technological advances required for reliable deployment, mission control, performance, and recovery of AUVs have developed considerably over the past 10 years. Currently, there are several vehicles operating successfully in the offshore industries as well as in the applied and academic oceanographic sciences. This article reviews the application of AUVs to fisheries- and plankton-acoustics research. Specifications of the main AUVs currently in operation are given. Compared to traditional platforms for acoustic instruments, AUVs can sample previously impenetrable environments such as the sea surface, the deep sea, and under-sea ice. Furthermore, AUVs are typically small, quiet, and have the potential to operate at low cost and be unconstrained by the vagaries of weather. Examples of how these traits may be utilized in fisheries-acoustics science are given with reference to previous work in the North Sea and Southern Ocean and to potential future applications. Concurrent advances in multi-beam sonar technology and species identification, using multi-frequency and broadband sonars, will further enhance the utility of AUVs for fisheries acoustics. However, before many of the more prospective applications can be accomplished, advances in power-source technology are required to increase the range of operation. The paper ends by considering developments that may turn AUVs from objects sometimes perceived as science fiction into instruments used routinely to gather scientific facts.