Marine Policy 64 (2016) 46-54 Contents lists available at ScienceDirect
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- Bu səhifədəki naviqasiya:
- Weather conditions
- Catch utilization
- Fishing Exploitation
- Market Regulations Social norms
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expect to optimise the species and length composition of the catch with respect to market demand and the fishing business efficiency. The lack of markets for many parts of the catch constitutes a major incentive to discard what does not match an established consumer demand. To reduce the work load of sorting the catch, fishers probably took early voluntary measures to limit bycatch, thereby improving fishing selectivity [1]. Expectations from the general public might be more related to the supply of food – including the supply of protein to ensure food security for a growing humanity ENVIRONMENT:
FISHER’S TACTIC & 
STRATEGY: Target species Characteristics of FOs: depth, duration, time, etc. GEAR: Design and size Mesh size Selective devices
have grown and could have been a valuable catch later on [2]. But discards are not necessarily lost to the ecosystem. First, even if discard mortality is believed to be generally significant [5,59], some discarded individuals might be able to survive. Even if dead when returned to sea, discards were proven to benefit spe-cies such as seabirds [62] and benthic communities [28,51]. Si-mulations of an ecosystem model suggest that completely elim-inating discards by landing all individuals that are currently dis-carded could cause trophic cascades, resulting in biomass reduc-tion of several food web components such as carnivore and scavenging benthos, birds and mammals [34]. Avoiding the un-wanted catch in the first place, by improving fishing selectivity, could benefit to the ecosystem under heavy exploitation, but less so under light exploitation [34]. The recycling of discards by marine ecosystems and their impact on food webs remain largely unknown, and require further research. 4. Factors and management tools Different actors in the system can handle different tools (Fig. 2) to influence progress towards selectivity-related objectives. Fish-ers directly implement fishing selectivity, but the outcomes at the ecosystem and society levels also depend on many other factors, some of which can be managed. 
General public Conservationists Values Catch utilization Consumers Markets Regulations
Marine community Fig. 2. Schematic view of the natural and human parts of the socio-ecosystem, of their main actors or components and of their relationships pertaining to fishing selectivity (dark grey arrows), exploitation pattern (grey arrows) and catch utili-sation (light grey arrows); adapted from [22].
Market Regulations Social norms DISCARDS or LANDINGS Fig. 3. Main factors affecting fishing selectivity (underlined), catch utilisation (bold) and exploitation patterns (italicised). Links between factors are dis-tinguished between direct links (full arrows) and influencial links (dashed arrows). FO stands for fishing operation. 4.1. Many manageable factors at the Fishing Operation scale Fishing selectivity can be modified by (i) fishing gear technol-ogy, and (ii) the detailed settings of the fishing operation, which we designate as fishing tactic below. Developments in gear tech-nology aim at improving gears by adapting their geometry, ma-terials and the way they are rigged. Fishers decide on the target species or species group according to markets and the availability of resources in a given season and area, contingent on regulations. The most appropriate gear and its specifications are chosen to meet this target according to availability and behaviour, and technical regulations, such as minimum mesh size. Factors related to the fishing tactic such as time of day, fishing depth and duration are also adjusted appropriately. The combination of gear char-acteristics, fishers' tactics, and conditions of the fishing operation, including meteorological conditions, determine the species and size composition of the catch (Fig. 3). Fishing selectivity depends on the interaction between the fish and the gear under the conditions of the fishing operation (Fig. 3). The factors affecting contact selectivity differ from those affecting available selectivity (Table 2). Contact selectivity mainly depends on the interaction between fish morphology and gear character-istics. Mesh size regulations are a much employed technical measure to let small fish escape once they contacted the gear, although their efficacy has been questioned [29]. Selective devices and other approaches were developed to allow undesired species to escape from the gear once they contacted it, based either on their morphology, such as turtle excluder devices – grids which direct individuals larger than the bar spacing out of the gear [30], or on their behaviour, e.g. dorsal square mesh panels which let hake escape from Nephrops trawls thanks to their upward swim-ming behaviour [26]. Other selective devices are designed to prevent contact with the fishing gear, e.g. acoustic devices to re-peal cetaceans [25]. The latter devices affect the available se-lectivity. Available selectivity primarily depends on the interaction between the behaviour of individuals in the area surrounding the gear and the way the gear is deployed. It can also be indirectly influenced by environmental factors affecting the swimming ca-pacity of the fish, the gear motion and the detectability of the gear by the fish ([33,63,64]; Table 2). Water turbidity, and mesh size and colour, for instance, influence the ability of fish to visually detect and avoid the gear [65]. In the case of towed gears, available
Table 2
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