Although longlines and gillnets may be considered as simple gears their fishing performance can be considerably modified by varying a number of parameters. For a researcher considering working with longlines and gillnets, it is beneficial to have some understanding of the importance of the major parameters. This section attempts to give a brief introduction to important gear parameters.
Hamley (1980) suggests the general rule: that the researchers use the technologies applied by the local commercial fishermen. The fishermen have usually accumulated and adopted a considerable amount of experience and are therefore usually up-to-date with technical development. The speed with which gillnet and longline fishermen have adopted new and superior technologies was experienced when the nylon gillnets replaced the previously used natural fibres in the 1950s (Hamley 1975) and when the new circle hooks were introduced in the longline fisheries (Quinn et al. 1985).
For research gears an uncritical copying of the technologies used in the commercial fisheries may, however, be unwise. Fishermen optimise their behaviour to achieve goals that differ from those of a researcher. For instance, gillnets and longline will most often be used for targeting relatively large individuals. Hovgård (1996a) and Erzini et al. (1996) note that applying large mesh-sizes or hook-sizes as used in the commercial fisheries may impede the estimation of reliable selection curves. Also, with regard to the costs, durability and the ease of work differences may be found between fishermen and researchers.
Gillnets are marketed in a variety of colours and shades and individual fishermen often show strong preferences for certain colours. Despite the individual variations a general trend is often observed. In the Danish fisheries, for instance, orange coloured nets dominate in the Baltic Sea whereas grey or green nets are preferred for the North Sea fisheries.
Baranov (1948) offered the simple hypothesis that the most efficient colour of nets should be that of the dorsal region of the fish as this colour conceals the fish in its particular environment. This should argue for darker nets being most efficient. Later studies (e.g. Jester 1973, Tweddle and Bordington 1988) have not confirmed Baranov's camouflage hypothesis as white nets often perform better than dark ones. In both these studies, clear species differences were observed such that the ‘best’ colours differed from species to species. Jester (1973) notes that the efficiencies of different colours also show seasonal differences.
Wardle et al. (1991) discuss the visibility of monofilament nets from a physical perspective and show interesting patterns regarding the importance of object orientation in water as well as the differences in reflection in air and water. They further inferred that the visibility of nets depends on both the water colour and the colour of the seabed. Cui et al. (1991) used light intensity thresholds as an indicator of the fishing power of different coloured nets by comparing mackerel (Scomber scombrus) behaviour towards different coloured twine. Considerable differences in colour detection thresholds were observed for both a thin monofile thread and a thicker multifilament thread
Although much remains to be done in the evaluation of the importance of the colour of gillnets, it is clear that considerable gains in catchability can be obtained by choosing an appropriate colour when targeting specific species. The results from Jester (1973) and Tweddle and Bordington (1988) indicate that the catch rate differences between the most and least efficient colours may exceed a factor of two.
It is a well-known fact to fishermen and net manufacturers that nets constructed of thinner twine catch considerably more fish than nets made of coarser materials. Fishermen usually attribute the higher fishing power of finer nets to these nets being ‘softer’.
The available experimental information suggests that the effect of twine thickness is found for all materials, i.e. multifilament (Predel 1963), monofilament (Jensen 1995a, Hovgård 1996b) and multi-monofilament (Steinberg 1985). An example of differences in fishing power of monofilament gillnets using twine of different dimensions is shown in Figure 7.2.
Baranov (1948) also offered the simple hypothesis that the effect of twine dimensions could be related to fishing power (P) by: P α (mesh-size / diameter). However, more comprehensive empirical work is needed to estimate the exact relation between fishing power and twine dimension (Hovgård 1996b).
The choice of dimensions of netting material implies a trade-off between fishing power and net durability as nets made of fine, materials are more easily damaged. In commercial fisheries, durability and ease of handling are often the main arguments to use relatively coarse netting materials. In research, where the cost associated with net damage is typically low compared to vessel and crew costs, it may generally be suggested to use nets of finer material than those used commercially.
Fishermen have clear notions of the importance of the material type where considerations are given to both catch performance and physical attributes. Multifilament nets (MF) are considered to be the least efficient while at the same time being the strongest. Multi-monofilament nets (MM) are generally considered to be the most efficient as the use of thin parallel threads make the net more ‘soft’ than the monofilament (MO) or multifilament (MF) nets. A stated draw-back of multifilament nets is the higher tendency for entangling various unwanted by-catches, e.g. crabs or starfish, which may considerably slow down the cleaning process.
The different qualities of the netting materials often led to clear patterns in their use. In Denmark, for instance, multifilament nets are typically used in the trammel nets targeting flatfish, multi-monofilament are the predominantly used materials when targeting cod, whereas hake fisheries most often use monofilament nets.
Most scientific comparisons have been between monofilament and multifilament nets. The results of these comparisons are not clear. Some studies, (e.g. Predel 1963, Washington 1973) find multifilament superior to monofilament nets whereas other studies indicate the opposite (e.g. Hylen and Jacobsen 1979). Studies including several species indicate that the differences may be species-dependent. Jester (1973) observes equal total catches between monofilament and multifilament nets
whereas clear differences are seen between species. Henderson and Nepszy (1992) find highest total catches in monofilament nets, but also that catches of 7 out of 23 species were highest in multifilament nets. Machiels et al. (1994) found monofilament nets more efficient for pikeperch (Stizostedion lucioperca), but multifilament more efficient for bream (Abramis brama).
Figure 7.1 Comparisons between the fishing power of multi-monofilament and multifilament trammel nets used simultaneously in sole fisheries in the Bay of Biscay. For mesh-size 8.4 cm the multifilament net caught slightly more fish than the multi-monofilament nets. For the 9.0 cm mesh size the multi-monofilament net catches were about double those found in the multifilament nets. Data: courtesy of J.-P. Brabant and J.Sacchi, IFREMER, France.
Stewart (1987) compared nets used in British cod fisheries and found that the multifilament net has a better catch than multi-monofilament and monofilament nets. The catch differences could be attributed to the way fish are enmeshed, whereby the multifilament and the multi-monofilament nets caught considerably more entangled fish. Stewart (1987) notes that the monofilament net was hard and springy whereas the other two materials were characterised as softer.
Lack of good understanding of the exact importance of material coarseness within the various materials complicates comparisons between materials. When comparing an inferior version of one material to a superior version of another material the catch differences will not only reflect the material. In an experimental fishery in the Bay of Biscay, using trammel nets targeting sole (Solea solea), multi-monofilament nets were generally found to be more efficient than multifilament nets (EU 1997). However, due to a limited availability of the various netting materials the two different net series could not be fully standardised and considerable differences were observed in the catches between equal mesh-sizes of the two materials (Fig. 7.1).
Commercial nets typically have a hanging ratio between 0.25 and 0.65. In the European marine fisheries the lower hanging ratios are applied in flatfish fisheries whereas nets for catching roundfish typically have hanging ratios between 0.4 and 0.5.
Research results have indicated that the hanging ratio may affect the selectivity of the nets. Riedel (1963) noted that a decrease in hanging ratio resulted in an increasing number of mainly smaller Tilapia mosambica becoming entangled in the net. Similarly, Engås (1983) found a decrease in selectivity for blue ling when the hanging ratio decreased from 0.6 to 0.4 due to more small specimens being caught in the loosely hanging nets.
Mohr (1965) found that the numbers of large perch increased when the hanging ratio decreased, whereas no change could be observed in the length frequencies for roach (Rutilus rutilus) when changing the hanging ratio. He interpreted the species difference as being due to the fact that the perch were a spiny fish that were entangled more easily when the hanging ratio was decreased. This interpretation, which has been widely cited, has recently been questioned by Machiels et al. (1994) who found a change in selection for the ‘smooth’ bream (Abramis brama) but not for the ‘spiny’ pikeperch (Stizostedion lucioperca).
Samaranayaka et al. (1997) noted a small increase in the amount of tuna entangled when the hanging ratio was decreased from 0.6 to 0.5.
Several studies (e.g. Samaranayaka et al. 1997. Machiels et al. 1994, Angelsen et al. 1979) report a general higher catch for the more loosely hung nets. Experimental fisheries carried out by the Danish Fisheries Research Institute suggest, that the catches of the dab (Limanda limanda) were highest in loosely hung nets (Fig 7.2) whereas little difference was seen for cod (Gadus morhua).
Figure 7.2 Effect of hanging ratio in coastal Danish fisheries for dab (Limanda limanda). The catches were taken in parallel sets using two different monofilament gillnets with twine diameters of 0.16 and 0.28 mm, respectively and with a mesh size of 26.5 mm. Both net types were rigged at three different hanging ratios, 0.2, 0.4 and 0.6. Note the decrease in catch with increasing hanging ratio and the difference in fishing power between nets of different twine diameters. (Courtesy of Mr J. Støttrup, DIFER, Hirtshals.)
Trammel nets are gillnets of a special design constructed by joining three parallel sheets of netting where the two outer sheets are made of netting with very large mesh-sizes. The length and height of the net are determined by the hanging ratio applied to the two outer sheets. The middle sheet is very loosely hung allowing bags of this netting to be drawn through the larger mesh-sizes of the outer net sheets.
The trammel net design enables the catch of fish by two different processes.
gilling and entangling as known for conventional gillnets, and:
catching of large fish taken in the bags of the inner netting.
The existence of the latter catch process suggests that a larger amount of bigger fish should be taken. Consequently, trammel nets should be less size selective than conventional gillnets.
The selectivity of trammel nets may be evaluated by the procedures used for conventional gillnets (Sacchi 1988, Holst and Moth-Poulsen 1995). Selection curves for conventional gillnets and trammel nets are compared for cod, plaice and sole in EU (1997). For fish sizes below the modal size little difference was seen between the selection estimated for trammels and conventional gillnets. For large cod and sole there was some indication of a reduced selection in the trammel nets whereas little difference was found for plaice (Fig. 7.3).
Figure 7.3 Selection curves for plaice (Pleuronectes platessa) estimated for trammel nets and for conventional gillnets. The trammel net selection curve was estimated for fisheries targeting plaice. The conventional gillnet selection curves were estimated from by-catches of plaice taken in fisheries aiming for cod and sole. Data from EU (1997).
When selecting nets to be used for research purposes the scientist should obviously attempt to choose as efficient a gear as possible, thereby increasing catches per research effort. Information on gear efficiency may be available from the fishing community, but the researcher may consider using other materials and finer twines than those commercially used. When several mesh sizes are to be used these should be standardised with respect to colour and twine quality. Twine dimensions should increase with mesh size attempting a fixed ratio between mesh size and twine diameter.
Both indirect selection studies and stock survey programmes require the simultaneous use of several different mesh-sizes. The actual choice of mesh-sizes to be included in the experiment net series depends on which of these two objectives are the focus.
126.96.36.199 Design of Net Series Used for Stock Surveys
The mesh sizes included in net series used for stock surveying should be selected to enable full coverage of the potential size distribution of fish that may be encountered. This may imply using a high number of different mesh-sizes — more than 10 mesh-sizes are often used (e.g. Degerman et al. 1992, Henderson and Nepszy 1992). The catches in mesh sizes matching the most abundant size groups may be high whereas other mesh sizes (mainly the larger mesh) may catch few fish. In some studies the amount of netting suitable for catching the most abundant size groups has been reduced (e.g. Regier and Robson 1966).
When estimates of the selection curve are available, it is possible to select the sequence of mesh-sizes in such a way that the total mesh selection, exerted by the entire net series, is about equal for all size groups (see e.g. Jensen 1990). When size selection estimates are not available, some guidance on the mesh size selection may be derived by using simple selection proxies. For example, the morphometric approach (Section 4) or using length frequencies in commercial catches (Section 5).
The sequence of different mesh sizes should be randomised between the individual net series used in the investigation. The different mesh sizes may be in the form of small nets that are tied together or as multimesh nets where the netting is mounted on the same head and foot rope. Kurkilahti and Rask (1996) remark that the latter design may be easier to use and be more cost effective.
188.8.131.52 Design of Net Series Used for Selection Studies
For selectivity studies the full size range of the fish population need not be covered and fewer mesh-sizes are therefore necessary in the research gillnet series. Typically, five to seven different mesh sizes are applied in this type of study. The sequence of different mesh sizes should also be randomised.
High catches are restricted to the size interval found between the modes of the smallest and the largest mesh size used in the net series. This interval is termed the ‘selection window’ (Hovgård et al., 1999). When the selection is described in accordance with the principle of geometric similarity (see Section 3) the selection window may be expressed on the l/m scale as:
where k is the selection factor and R is the ratio between the largest and the smallest mesh size used.
The selection window formulation illustrates that selection is well described in the neighbourhood of k, but that the data available for estimation is poor for relative fish sizes R times above or below k. This implies, for instance, that a researcher who wishes to discern a secondary mode in a bi-modal selection curve with the mode expected at 1.5 times the primary mode must use a range in mesh sizes above 1.5.
In actual work high catches may not be found in the entire selection window. This happens when the mesh sizes used do not match the size distribution fished leading to insignificant catches in some, typically the largest, mesh sizes. Hovgård et al. (1999) provide an example of the ambiguity in the description of selection curves caused by this phenomenon. The implications for the experimental planning is that appropriate mesh sizes can only be chosen when some notion of the population size structure is available.
For indirect selection studies, the multimesh type of net series used by Kurkilahti and Rask (1996) cannot be recommended. In these nets, the different mesh sizes are stitched together. In consequence, fish that are too large to be caught in a mesh size may be led to a more appropriate mesh-size by the net wall. This violates the fundamental assumption of indirect estimation, viz. that a fish of a particular size has an equal probability of encountering all different mesh-sizes.
Hooks may be differentiated by a multitude of parameters such as their general form (various models of both the traditional J-shaped and modern circle-shaped hooks); their size as measured by gape width, shank length and wire dimension; material (e.g iron, stainless steel); the shape of the point (e.g. with/without barb, the way an edge may be cut); the form of the eye (e.g. a loop or a flat plate) and finish (e.g. colour, coating etc). Hooks are often also available in flat and twisted models. Special shapes of hooks have been constructed to comply with the different storage and baiting devices used in the various automated line systems introduced in the last few decades. The number of different hook types appears to be very large.
The traditional ‘J’ shape hooks have in many fisheries been changed to types of ‘circle’ hooks. The circle hook has proven to be clearly superior to the traditional ‘J’ shaped hooks in a number of fisheries (Forster 1973, Skeide et al. 1986, Quinn et al. 1985). Bjordal (1989) infers that the higher fishing power of the circle hook designs are caused by both a higher hooking efficiency and a lower level of escapement of hooked fish. The higher hooking efficiency of circle hooks has been related to the pull exerted by a caught fish as being directly in the direction of the hook eye.
The size of the hook will influence the selective properties as described in Section 5, but will also be of importance with respect to the breaking strength.
Erzini et al. (1996) make the observation that a high number of hook experiments show a decreasing fishing power with increasing hook size. The work by Johannesen (1983) may explain in part this phenomenon as hooks made of finer wire diameter more easily penetrated the fish mouth tissue than did coarser hooks.
Most fishermen consider bait probably the most important single factor for enhancing longline catches and they often have good experience as to what kind of bait is particularly suited to their target fishery. However, bait is a very significant cost component in many longline fisheries and the actual choice of bait is therefore often a trade-off between bait quality and cost. In some fisheries, it is customary to use a less valuable bycatch species for bait (e.g. in the Pacific and Atlantic halibut fisheries, Hamley and Skud 1975, author's observation). In many fisheries it is customary to use a combination of several bait types concurrently (e.g. squid and mackerel in some North Atlantic fisheries). Live bait, has been used in several traditional fisheries and seems to be very efficient (author's observation), but its use is now forbidden in many countries for animal rights considerations.
Comparisons of catch differences between bait types have shown that squid and octopus are often superior to fish bait (e.g. Martin and McCracken 1954, Hamley and Skud 1978, Bjordal 1983a). This may to some extent be attributed to the durability of these bait types at sea. This has been shown by underwater observations where squid and octopus are found to be less easily removed from the hook than fish (High 1980, He 1996). Lower bait loss of squid than of fish are also observed from retrieved longlines (Bjordal 1983, He 1996).
The attractiveness of bait has been related to bait quality. For instance, pre-soaked bait, where attractants have been washed out, have shown a poorer catching performance than fresh bait (Løkkeborg and Johannessen 1992). Faeroes experiments using bait of different age and fat content have similarly shown the best catches to be the fresh bait with high fat content (J. iHjaltostovu pers. comm.).
In traditional longlines the main line and the gangions were made of various types of multifilament ropes (nylon, polyester, polypropylene). The main line is typically of a diameter of 4–12 mm whereas the gangions are considerably thinner.
In several longline fisheries, transparent monofile lines have replaced the traditional materials, which has typically lead to increased catches. The monofile line is stronger allowing the lines to have a thinner diameter. The higher efficiency of the monofile lines may at least partly be due to the lower visibility of the thin monofile line. Bjordal (1983b) found that catch rates were 10–20% higher when traditional multifilament gangions were replaced by monofile gangions.
Figure 7.4 Reducing the effect on unwanted predators (birds or scavengers) by regulating the longline setting depth through choosing material of different buoyancy. The pelagic longline is kept below the surface by materials with negative buoyancy. The demersal longline is moved off bottom by using materials of positive buoyancy. More elaborate rigging schemes allowing various depths to be fished are found in Sainsbury (1996).
As the buoyancy of line materials differs, the choice of materials allows the setting of lines at various distances from the bottom or the surface (Fig. 7.4). Moving a demersal line just off bottom or a pelagic line well below the surface provides a simple mechanism for reducing the bait loss caused by sessile scavengers or birds respectively.
He (1996) compared the bait loss from floating (polypropylene) and sinking (polyester) bottom set longlines and found a significant lower bait loss for the line lifted off the bottom. Bjordal (1983b) who lifted hooks off the bottom by attaching small floats to individual hooks observed a similar reduction in bait loss.
The spacing between gangions differs considerably in commercial fisheries depending on the species targeted. High spacing is typically applied in fisheries targeting large and valuable species found in relatively low densities (e.g. tuna, salmon and halibut). In these fisheries gangion distances are typically 10 and 50 m (Skud 1978a). Much lower distances are used in fisheries targeting mixtures of smaller species. A typical distance in North Atlantic fisheries of this kind is about one fathom (1.8 m).
Hamley and Skud (1975) found that the catch per hook increased with increasing gangion spacing, which may be interpreted as due to a larger area being affected by the odour plume from the bait when hooks are well spaced. Also the proportion of halibut was higher at increasing gangion distance which Hamley and Skud interpreted as due to a between-species competition for bait.
The importance of gangion length and attachment has been evaluated in various Norwegian experiments. Karlsen (1976) found a considerable reduction in catches of tusk and ling when shortening gangion length from the conventional length of 40 cm to 15 cm. Bjordal (1987) noted that attaching gangions to the main line by swivels, instead of the conventional attachment by a knot, increased catches by about 15%. The reasons why swivel type and gangion length affect catch performance have been related to twisting of the gangion around the main line during hauling — thus causing fish to be pulled off the hooks (Bjordal 1989).
The researcher should take advantage of the improvements in longline technology that have occurred in recent years. Sainsbury (1996) summarises the potential improvements in catch rates as: change of the conventional ‘J’ shaped hook to the circle hooks 15–20%; use of monofile material for gangions 10% and attaching gangions by swivels 15–20%. If longlines are to be part of ongoing stock surveillance programmes consideration should be given to equipping research vessels with mechanised line setting systems. In Greenland this has allowed the daily survey effort to be increased four-fold in abundance surveys for Greenland Halibut (J. Boje pers. comm.). The researchers should also consider using other bait than that used in the commercial fisheries as the commercial bait-choice may be based on some trade-off between efficiency and cost.
As noted by several researchers (reviewed by Løkkeborg and Bjordal 1992), the importance of hook size may be less than that of bait size. When wanting to avoid confusing these two effects, Otway and Craig (1993) stressed the need for standardising bait size. Punt et al. (1996) accept the confusing effects between hook and bait sizes and note that hook and bait size are usually correlated in commercial fisheries. They therefore implicitly treat the hook-bait combination as one entity. If the bait size influences the affected area this approach cannot be recommended for selection studies. Strict standardising of the bait sizes is recommended.
The few longline stock abundance surveys, which have been carried out, have used a single hook size only. A single gear size may be more appropriate than for gillnets due to the less pronounced selectivity of longlines (see Section 5). However, for experiments where longline catch size distribution is compared with trawl catch information, it appears that the catch efficiency of the smaller individuals has been unsatisfactorily low. The researcher should therefore consider using several hook-sizes concurrently to achieve a better size coverage. It is generally recognised that a considerable contrast in hook sizes is required to statistically distinguish differences in the size frequencies of catches taken on the different sizes of hooks (e.g. Ralston 1982, Bertrand 1988, Otway and Craig 1993, Erzini et al. 1996). Based on these works a minimum size difference between adjacent hook sizes should be approximately 1.4 times the size.