El traslado de los “invasores” sin techo: de Pamplona a los arenales de la Tablada de

The range of habitats used by lobsters creates difficulties when conducting surveys of abundance. For complex inshore habitats, diver surveys are often

considered the best approach (Pitcher, Dennis et al. 1997), but are restricted by depth, time, cost and environmental conditions. Trawling is sometimes used in low complexity habitats, but is easily avoided by lobsters (Roddick and Miller 1992). Most lobster fisheries deploy stationary baited parlour traps that attract animals (Fig. 1.4). Used extensively in commercial shellfisheries, and as a tool for population studies, traps are

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relatively inexpensive, can be deployed from small vessels in any habitat and in any configuration, with relatively little damage to catch or habitat compared to mobile gear. However, indices of abundance are based on the assumption that the catch of the trap is representative of the surrounding population.

Previous studies have demonstrated that the catchability of an individual animal depends on size, sex, moult status, and environmental factors such as water temperature and currents. Miller (1990) has provided a comprehensive review of factors governing trap efficiency. Understanding trends in catchability is fundamental to improved assessments of stock status that rely on trap data (Tremblay and Smith 2001), but catch rates are subject to uncertainties due to additional factors, such as escapements, gear design (Montgomery 2005), selectivity and saturation effects, species interactions, changing area of bait influence or attractiveness and seasonality (Bennett 1974a; Miller 1990; Fogarty and Addison 1997; Bell, Addison et al. 2001;

Ziegler, Frusher et al. 2003). Often summarised as catch probability and effort, it is important to understand how external factors influence observed catch, so that data can be standardised and more representative of the abundance of the target species.

1.4.1 Seasonality

Homarus gammarus catch per unit effort (CPUE) is relatively low, previous studies estimate it at 1% that of Cancer pagurus (Bennett 1974a), with two seasonal peaks generally observed; there is a spring peak, lasting three to four months, when effort changes to target lobsters inshore and rising water temperature increases lobster

Figure 1.4 Standard design of a parlour trap.

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activity, and a shorter peak in the autumn, possibly following the emergence of newly moulted lobsters, including those recruiting into the fishable stock for the first time (Fig. 1.5). This seasonal pattern is reflected in the majority of UK shellfish studies, landing data and anecdotal evidence, and must be taken into account during fishery-independent studies or when using commercial catch data.

1.4.2 Soak-time

Over the past 40 years, the effect of soak- or immersion-time on CPUE has been studied by several authors (Bennett 1974a; Montgomery 2005), but expressing this process quantitatively is difficult. The process of trap saturation, the reduction in catch rate with increasing catch (Miller 1979), has long been recognised, and a number of models have been developed to describe the process (Fogarty and Addison 1997).

However, gaining sufficient data by experimental fishing can be time-consuming;

therefore much of the available data are derived from commercial records.

Trap catches generally increase over the soak period, but do not necessarily increase linearly (Bennett and Brown 1979). Catch rates of Jasus verreauxi in New Zealand are not affected by soak-times between 1 and 3 days as traps often saturate within 24 hours (Montgomery 2005). However, traps do not have fixed saturation levels, but vary

Figure 1.5 Monthly changes in lobster catch per unit effort during 1971 (Bennett 1974).

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with season (Munro and Therriault 1983), choice of bait, target species, trap design and location. Most studies of saturation for C. pagurus, H. americanus and H.

gammarus, found that catch rates begin to plateau after 24 hours (Dow 1961; Bennett 1974a; Fogarty and Borden 1980; Fogarty and Addison 1997). At high densities,

saturation could limit catches in less than 12 hours (Miller and Rodger 1996), but as few fisheries lift traps more frequently than daily, and as lobsters feed more actively at night, greatest catches are generally obtained after soaks of 24 hours. In some

instances, after an initial decline in rate of catch increase, a second increase in catch may be seen after 4 or 5 days. This could be due to escapements making the trap attractive again, or animals within the trap dying, acting as fresh bait (Breen 1989). In addition, with extended soak times the attractiveness of the bait diminishes, resulting in reduced catch rates. Conversely after four or five days the reported increase in catch rate may be due to the decomposition of the bait and further release of attractive substances.

The swimming ability and manoeuvrability of lobsters, which is much greater than that of crabs, can allow for easy escape from traps, particularly those with ‘hard-eyed’ or

‘fixed’ entrances. Jury et al. (2001) analysed videotapes to reveal that traps caught about 6% of lobsters that entered; allowing 94% to escape (Jury, Howell et al. 2001).

This high rate of escapement means that the observed catch of a trap is only the catch at the time of hauling, and not necessarily representative of the animals that have entered the trap over the course of the soak.

1.4.3 Effective area fished

When estimating abundance or density by any method, it is essential to know the area of habitat being sampled and the efficiency with which individuals are detected within this area. Unlike direct sampling devices such as quadrats that are characterised in terms of area or volume covered per unit, sampling properties of baited traps are not easily estimated. The key property is the effective area fished, which is the notional area of seafloor containing as many animals as were trapped (McQuinn, Gendron et al. 1988; Miller 1989). It can be defined as a catchability coefficient, allowing for the conversion of CPUE to population density (Miller 1990).

However, effective area fished is also the most difficult property to measure (Bell,

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Addison et al. 2001). Animals are attracted to traps by the bait odour plume

(Reidenbach, George et al. 2008; Reidenbach and Koehl 2011), therefore, independent of interactions or trap spacing, the shape of the fished area will be dictated by water currents, foraging behaviour and seafloor topography, and constrained by certain habitats or obstructions (McQuinn, Gendron et al. 1988; Watson, Golet et al. 2009).

The difficulties of estimating the area being sampled by baited traps causes it to be overlooked in the majority of trap-based studies, but can have dramatic effects, particularly when converting abundance to density (Bell, Eaton et al. 2003).

1.4.4 Species interaction

In most trap fisheries several species are caught by the same gear and competitive interactions inside and outside traps are likely to influence the capture process and affect ingress and egress (Rossong, Williams et al. 2006; Williams, Floyd et al. 2006). Interactions between individuals of the same species (intra-specific) and different species (inter-specific) influence the catchability of portions of the population (Miller 1979; Richards, Cobb et al. 1983). Reduced entry of Cancer productus and Cancer magister has been linked to behavioural interactions causing significant reductions in catch (Miller 1979); in addition avoidance of dead conspecifics can also alter catchability (Richards, Cobb et al. 1983; Addison 1995).

Agonistic interactions between H. gammarus within a trap and animals approaching reportedly inhibit catch rates of both H. gammarus and C. pagurus (Addison 1995). The presence of one or two H. americanus in a trap also significantly reduced subsequent catch rates of both conspecifics (Smolowitz 1978) and Cancer spp (Richards, Cobb et al.

1983). In particular, aggressive intraspecific interactions over control of the bait, appear to be the dominant factor limiting both rate of entry and rate of escape of H.

americanus (Jury, Howell et al. 2001).

Without understanding the relationship between trap catch and the population present around the trap, assessments based on catch data will have unknown biases.

Catchability, variable both seasonally, temporally and between portions of the population (Dunnington, Wahle et al. 2005), can also be influenced by physiological, behavioural and environmental factors and variations in gear design. Therefore, numbers and distributions of animals among traps may not represent the relative