Small-scale fisheries (SSFs) are strongly represented in all European Union (EU) member states (Guyader et al. 2013), so many coastal communities rely on fisheries and fishing-related activities as a means of subsistence, income and employment (Oliveira 2014) that has created an ancient cultural heritage and socio-economic dependence. In Portugal, the bivalve dredging fleet is of great importance among the SSFs, accounting for a large number of vessels and many fishermen, a high volume of catches and high value of the products (Oliveira 2014). The vast majority of the Portuguese fishing fleet is composed of artisanal vessels (Oliveira et al. 2007), and the largest bivalve dredging fleet is based on the Algarve coast (southern Portugal), with 53 vessels operating an average of 177 days year^–1 (DGRM 2015).

In Portugal, bivalve dredge fisheries have been performed since 1969 (Chícharo et al. 2002a). On the Algarve coast, the dredging fleet targets mainly three commercially valuable bivalve species: Spisula solida, Chamelea gallina and Donax trunculus (Gaspar et al. 2015). The surf clam (S. solida) occurs mainly between 3 and 14 m depth; it is the most abundant bivalve species in this area and has shown increasing densities over the last decade. The striped venus (C. gallina) has a similar depth distribution, being found mainly from 3 to 10 m depth (Gaspar et al. 2015). The donax clam (D. trunculus) occurs within a narrower bathymetric range (0-5 m depth, with higher densities at 3 m depth) (Gaspar et al. 2015), but it is among the most important molluscan species commercially exploited in the southern Iberian Peninsula and western Mediterranean Sea (Gaspar et al. 1999, Tirado et al. 2011).

The mode of operation and technical design of mechanized dredges have undergone gradual developments thanks to studies aimed at improving dredge selectivity (Gaspar 1996, Gaspar et al. 1999, 2002a) and reducing catch mortality (Gaspar and Monteiro 1999, Gaspar et al. 2003a, Leitão et al. 2009). The most recent modification to the dredge design included the adoption of a metallic grid box to retain the catches above the target species minimum landing size (MLS), by filtering out smaller individuals of both target and non-target species. Though it is more efficient and selective than previous dredge designs (Gaspar et al. 2001, 2003a, Leitão et al. 2009), in some periods of the year the amount of bycatch collected by this gear can exceed the catch of the target species, especially in late spring and early summer (Gaspar and Chícharo 2007).

The bycatch (capture of non-target organisms) includes incidental catches, non-target catches retained by fishermen, and discards, i.e. catches returned to the sea (McCaughran 1992, Alverson et al. 1996) because they are unmarketable species, highly damaged specimens or individuals below the MLS (Kelleher 2005). By changing the species relative abundance, together with the population structure of prey and/or predators, bycatch may lead to structural and functional disturbances in the ecosystem (Pauly et al. 2002, Thrush and Dayton 2002). For instance, bycatch can modify the diversity, biomass and productivity of the associated biota (Jennings and Kaiser 1998); disrupt trophic interactions (e.g. removal of prey and/or predators) (Crowder and Murawski 1998, Pauly et al. 1998) with subsequent modifications to food webs (Gaspar et al. 2001); change the structure of benthic communities in the short and long term (Jenkins et al. 2001); alter species foraging behaviour (FAO 2003); reduce the ratio of large- to small-bodied species (Bianchi et al. 2000); and affect fishing yield in other fisheries (Clark and Hare 1998).

Discarding by bivalve dredging vessels would not constitute a major problem if the discarded individuals survive after returning to the sea (Gaspar and Chícharo 2007). However, it is crucial to analyse the composition of discards in order to propose new strategies to minimize their impact (Urra et al. 2017). Indeed, fisheries management policies recommend a reduction in the discards (Catchpole et al. 2005) and the adoption of market-based approaches to promote added value for bycatch species (Leitão and Baptista 2017). The reformed Common Fisheries Policy of the EU (EC 2014) imposes a discard ban for all species subject either to quotas or MLS, resulting in a mandatory landing obligation for all catches (Vogel et al. 2017). However, in certain circumstances, this regulation allows some discarding when scientific evidence demonstrates high probabilities of survival of the discards (Morfin et al. 2017).

In order to further assess fishing impacts, the survival of bycatch is often estimated through damage scales and survival experiments (ICES 2014). Because damaged individuals are more vulnerable to predation and disease, it is crucial to accurately evaluate their level of damage upon discard (Pranovi et al. 2001). Many studies on this topic have focused on bycatch and mortality resulting from fish and shrimp trawling (for a review, see Broadhurst et al. 2007), but few have dealt with discard mortality in bivalve dredge fisheries (e.g. Hauton et al. 2003, Leitão et al. 2009, Urra et al. 2017).

In the bivalve dredging fishery, the optimal situation would be one in which maximum efficiency, low bycatch of non-target species, retention of very few undersized individuals and a low proportion of damaged individuals prevailed as prerequisites to mitigate the effects of the discard ban or similar policies. In order to achieve these goals, management measures should be implemented and fishing gear modified according to the local environmental conditions, the population status of the target species and current discard rates. The present study aimed to 1) characterize and quantify the bycatch of the metallic grid dredge used in the fishery targeting S. solida, C. gallina and D. trunculus along the Algarve coast; 2) evaluate seasonal variations; 3) estimate damage and mortality by using damage scales and performing survival experiments; and 4) propose management measures to minimize both discards and mortality.

The assessment of bycatch issues is critical for assessing the sustainability of any fishery. In addition to determining the catch composition, the quantification of bycatch and discard rates allows the efficiency and operational performance of the fishing gear to be measured. The fact that previous studies recorded considerable amounts of bycatch in the bivalve dredge fisheries along the Algarve coast, even exceeding the catch of the target species in late spring and early summer (Gaspar and Chícharo 2007), reinforces the importance of the present study and the need for further investigation on this important subject. Other studies have reported seasonal trends in the bycatch collected by mechanical bivalve dredges: namely an increase in flatfish discards in autumn on the Algarve coast (Palma et al. 2003), possibly associated with greater abundances in benthic communities in this area during this season (Alves et al. 2003), and also higher discards of D. trunculus in spring and summer in the northern Alboran Sea (Urra et al. 2017).

In the present study, the quantification of bycatch using two grid dredges with different technical specifications revealed highly significant differences in the composition of the total catches between types of dredge that were mainly due to the abundance and biomass of the target species (S. solida, C. gallina and D. trunculus) and also of the hermit crab (D. pugilator). These clear differences confirm that each type of dredge was developed and is well adjusted to maximize the catch of its target species. Although significant, less evident differences were detected in the composition of the bycatch collected by the two dredges, probably because of the overall similarity of the benthic communities at closely located sampling sites and bathymetries. The bycatch composition was dominated by the presence of commercially undersized individuals of the target species and by benthic species with dimensions and morphological features that prevented their passage through the parallel bars of the grid dredge, such as larger bivalve species, hermit crabs and heart urchins. This is a common issue in fishing gears designed to mechanically preclude the catch according to specimen size. For instance, because smaller individuals escape through the interconnecting steel rings of the dredge, the bycatch composition of the king scallop dredge fishery in the English Channel is dominated by large benthic and demersal fish species (Szostek et al. 2017).

The bycatch rates represented 46.0% of total catches in abundance and 32.9% in biomass using DDredge. In the dredge fishery targeting C. chione performed in the Setúbal region (western coast of Portugal) a high proportion of bycatch (31% in number) was also recorded (Gaspar et al. 2001). Significantly lower bycatch rates (13.5% of total catches in abundance and 6.3% in biomass) were recorded for SDredge, due to high densities of its target species (S. solida) in the bivalve beds along the Algarve coast. Another study also using a grid dredge to target S. solida in the same sampling area reported a mean bycatch rate of 9% in biomass (Leitão et al. 2009). In addition, previous fishing surveys performed in the same area recorded much higher fishing yields of S. solida (957 g/5 min. tow) than of D. trunculus (248 g/5 min. tow), reflecting clear differences in species abundance and density over time (Gaspar et al. 2015). For the same reason but with an opposite trend, low densities of target species were responsible for very high bycatch and discard rates (≈90%) in the “rapido” trawl scallop (Pecten jacobaeus) fishery in the Adriatic Sea (Pranovi et al. 2001).

In the present study, damage and mortality, which reflect the direct impact of the dredging operations, were predominantly low despite the presence of some sensitive species in the catches. The three bivalve target species (S. solida, C. gallina and D. trunculus) exhibited low damage and mortality rates, S. solida being the least affected and D. trunculus the most affected by dredging impacts. Likewise, except for D. trunculus, which showed slightly higher mortality, the undersized individuals of the remaining target species also displayed fairly low damage and mortality rates. Overall, damage and mortality depend on the operational and technical characteristics of the fishing gears and on some biological and ecological features of the taxa collected as either target catch or bycatch. In particular, bivalve species susceptibility to damage are markedly influenced by their shell morphology and resistance to breakage (the force required to break the shell) (Bergmann et al. 2001, Vasconcelos et al. 2011). Accordingly, bigger damage rates and significantly higher mortality rates in DDredge than in SDredge are probably due to the lower proportions of thick-shelled and more resistant bivalves, such as S. solida and C. gallina, in the catches using DDredge.

In contrast, the pod razor shell (E. siliqua), a thin-shelled bivalve that usually burrows shallowly close to the sediment surface but can burrow down to 60 cm deep when disturbed (Gaspar et al. 1998), showed very high mortality rates independently of the type of dredge. Such high mortalities are explained by the difference between the dredge teeth length (20 cm) and the maximum burrowing depth of E. siliqua (60 cm), because the perturbation caused by dredging induces deeper burrowing as a self-protective behaviour. Most individuals are hit by the dredge teeth in the upper and middle portions of the shell, suffering severe damage and subsequent mortality (Gaspar et al. 1998). Though it is the third most abundant species caught by both types of dredge, the hermit crab (D. pugilator) showed high resilience to dredging impacts in terms of both damage rate (<10%) and mortality rate (<1%, mainly individuals that abandoned their protective shells). This low sensitivity, coupled with recruitment every four months on the Portuguese coast (Dolbeth et al. 2006), makes this species almost invulnerable to bivalve dredging impacts on the Algarve coast. Swimming crabs (Liocarcinus sp.) also occurred in high abundances but displayed low damage and mortality rates, corroborating previous data gathered using the metallic grid dredge (Gaspar et al. 2003b). However, physiological responses of Liocarcinus depurator to aerial exposure reveal that, although it is not directly lethal, the stress induced by emersion might lead to metabolic disruptions that increase the susceptibility to predation (Bergmann et al. 2001).

Damage and mortality rates of echinoderms confirmed their fragility when subjected to impacts and stress, with the four species of sea urchins (Echinocardium spp.) being highly susceptible to lethal injuries caused by both types of dredge. This previously reported high susceptibility to damage (e.g. Kaiser and Spencer 1995, Gaspar et al. 2003b, Leitão et al. 2009) was attributed to the fact that the fused plates of sea urchins imply low flexibility and high sensitivity to mechanical damage during bivalve dredging (Kaiser and Spencer 1995). By contrast, brittle stars (class Ophiuroidea) exhibited very high damage rates but very low mortality rates. Several studies have confirmed the high resilience of the serpent star (Ophiura ophiura) to bottom fishing, considering that arm damage and breakage induce low mortality due to the high regeneration capacity of this species (e.g. Kaiser and Spencer 1995, Hill et al. 1996, Ramsay et al. 1998), which also relies on its high reproductive resilience to cope with severe damage and consequent mortality (Pranovi et al. 2001). However, Bergmann and Moore (2001) highlighted that fishing-related stress and damage could induce higher susceptibility to bacterial infection and subsequent death in this species.

There is lack of information regarding the survival of bycatch species after discarding (Leitão et al. 2014). In the Portuguese dredge fishery, most discards are invertebrate species (bivalves, gastropods, crustaceans and echinoderms). Depending on the volume of the catches, the specimens caught are sometimes only sorted at the end of the fishing day, which may decrease the survival of discarded individuals (Gaspar and Chícharo 2007). Despite the overall low mortality recorded in this study, dredging-induced stress might cause a slow recovery of the activity of discarded individuals, which thus become more vulnerable to predation (Robinson and Richardson 1998, Chícharo et al. 2002b). However, several authors have reported low rates of indirect mortality in molluscs and crustaceans (Gruffydd 1972, Caddy 1973, Kaiser and Spencer 1995), the two most abundant taxa collected in the present study. Larger individuals in particular are typically more resistant to damage and mortality (Birkett 1959, Medcof and MacPhail 1964, Trewin and Welsh 1972).

In order to estimate survival rates more accurately, captivity experiments were performed in aquaculture facilities during the present study. However, it is assumed that the effect of captivity upon the experimental individuals might lead to an underestimation of survival rates, whereas the exclusion of predation effects might lead to an overestimation (ICES 2014). Based on the arbitrary damage scores used in this study, the high survival rate of undamaged (D₀) and slightly damaged (D₁) individuals allowed us to infer that in this bivalve dredge fishery discarding can lead to considerable survival rates when catches are sorted immediately after each tow and discarded near the natural beds of the harvested species. One interesting finding of the captivity experiments was the fact that, when subjected to the perturbation and stress during dredging, D. trunculus appears to be faster closing the valves than retracting the foot, so apparently undamaged individuals were actually injured internally, which might jeopardize their likelihood of survival.

The design of the currently used grid dredge allows smaller individuals of both target and bycatch species to escape through the spacing between bars of the grid. However, although the dredges are adapted to the bivalve target species, they retain larger specimens of the bycatch species, which are later hauled, sorted on board and discarded. However, this gear proved much better than the traditional and northern dredges, because it collected a significantly lower amount of unwanted accessory species (Gaspar et al. 2001, 2003b, Leitão et al. 2009). In a strictly scientific approach to the problem of bycatch, high bycatch rates should not be a major problem if most discarded individuals survive (Gaspar and Chícharo 2007), but bycatch is also an ethical, technical, political and economic issue (Hall et al. 2000).

The morphological diversity of the bycatch taxa caught by bivalve dredging requires a dredge design that is efficient, selective and causes low damage to all collected organisms. The current technical design of the metallic grid dredge should be basically maintained and only slight modifications should be introduced, namely by introducing a bycatch reduction device (BRD) in order to allow most bycatch specimens to escape. Despite extensive research confirming significant bycatch reductions due to the adoption of BRDs (e.g. Brewer et al. 1998, Fonseca et al. 2005), little information of this type is available for bivalve fisheries, since most studies have focused on prawn and shrimp trawl fisheries. In the present case, the grid dredge should include, in the middle of the collecting system, an oblique metallic grid ending at an escape exit at the top of the cage (Fig. 6). By installing this BRD, it is expected that individuals larger than the openings will be guided upwards to the escape exit, while smaller individuals will pass through the openings of the BRD. Since the selection of the individuals that pass through the BRD will occur inside the grid cage, the spacing between the bars must be large enough to enable target individuals to pass through and thus avoid negative affects on the fishing yield. This type of BRD (i.e. a simple metallic grid that mechanically excludes unwanted catches according to their size) has been extensively reported as efficient for allowing bycatch to escape while maintaining the target catch. For instance, a significant decrease in the proportion of bycatch fish in trawl fisheries targeting the ocean shrimp (Pandalus jordani) was recorded following the implementation of a rigid-grid BRD (Hannah and Jones 2007). Likewise, significant bycatch reductions were achieved thanks to the use of the Nordmøre grid in the Brazilian artisanal shrimp fishery, with an extremely high decrease (97%) in the weight of the abundant brachyurids present in the catches (Silva et al. 2012).

The survival of escaped organisms should also be considered, since the adoption of BRDs assumes that excluded individuals suffer negligible mortality (Crowder and Murawski 1998). Considering that previous underwater observations by Gaspar et al. (2001) revealed that undamaged individuals that pass through the parallel rods of the metallic grid dredge rebury immediately or recover activity, the likelihood of survival of the escaped individuals is presumably high. Therefore, this BRD is expected to reduce both direct and indirect mortality, since it will allow the immediate escape of larger individuals from the fishing gear during the tow. They will be subject to less stress, recover their activity faster and be less exposed to predation. However, the modification proposed might also decrease the fishing yield, since during hauling there is a high likelihood of losing the target catch through the opening at the top of the dredge. To overcome this problem, an additional modification is required in the fishing gear: removing the rear part of the grid cage and attaching a net bag to the rear of the dredge in order to retain the catches during hauling (Fig. 6).

Gear-based solutions for reducing bycatch require an optimal combination of characteristics that decreases the amount of bycatch while maintaining or increasing the catches of target species. The likelihood of fishermen accepting a new or modified fishing gear can be expected to be low if the fishing yield decreases in comparison with the previous design of the fishing gear. For instance, the decreasing fishing yield following the adoption of the Nordmøre grid in the crustacean trawl fishery in Portugal raised concerns about whether fishermen would accept this gear modification (Fonseca et al. 2005). The overall results gathered in the present study recommend technical modifications to the current technical design of bivalve dredges, in order to adopt a BRD and include a retention net bag. For this reason, further experimental fishing surveys (performed throughout the year, using dredges with and without BRD towed simultaneously and side by side) are required to compare the fishing yield and profitability, catch composition, proportion of bycatch and discards, damage and mortality rates.