Comparing the catch composition, profitability and discard survival from different trammel net designs targeting common spiny lobster (Palinurus elephas) in a Mediterranean fishery

Abstract

In the Balearic Islands, different trammel net designs have been adopted to promote fisheries sustainability and reduce discards. Here, we compare the catch performance of three trammel net designs targeting the spiny lobster Palinurus elephas in terms of biomass, species composition and revenue from commercial catches and discards. Designs differ in the netting fiber type (standard polyfilament, PMF, or a new polyethylene multi-monofilament, MMF) and the use of a guarding net or greca, a mesh piece intended to reduce discards. Catches were surveyed by an on-board observer from 1,550 netting walls corresponding to 70 nets. The number of marketable species captured indicated that the lobster trammel net fishery has multiple targets, which contribute significantly to the total revenue. The discarded species ranged from habitat-forming species to elasmobranches, but the magnitude of gear-habitat interactions on the long term dynamics of benthos remains unclear. No relevant differences in revenue and weight of discards were detected after Bayesian analyses. However, the species composition of discards was different when using greca. Interestingly, high immediate survival was found for discarded undersized lobsters, while a seven day survival assessment, using captive observation, gave an asymptotic estimate of survival probability as 0.64 (95% CI [0.54-0.76]). Therefore, it is recommended that it would be beneficial for this stock if an exemption from the EU landing obligation regulation was sought for undersized lobsters in the Balearic trammel net fishery.

Keywords: Catch composition; Discard; Discard survival; Guarding net; Revenue; Spiny lobster; Trammel net.

Figure 1. Details of the strip added at the bottom of trammel nets (greca).

Greca net approximately 20 cm of nylon material with a mesh size of 45 mm that is sown to the bottom of the main net along its entire length.

Figure 2. Principal components analysis (PCA) plot of all the small-scale boats from Majorca that targeted spiny lobster during the 2015 and 2016 spiny lobster seasons.

Between-boat similarity was estimated from the landed catches (kilogram of each one of 170 commercial categories) for all the daily fishing trips of the lobster trammel net fleet from Majorca. The gray points denote the average scores for all the boats (i.e., the centroid of all the fishing trips of a given boat). The larger, colored points denote the three sampled boats. For them, an envelope polygon including all the fishing trips of each boat was added. Note that within-boat variability is similar to or even larger than between-boat variability. The number on the axis label indicates the proportion of variance explained by each of the first two PCA axes considered.

Figure 3. Marketable catches and Discards.

(A) Number of items per netting wall of Marketable catches (blue bars); and Discards non-marketable species (red bars); (B) kilogram per netting wall of discards (non-marketable species), pooled for catch attributable at the netting wall/panel and whole net levels.

Figure 4. PMF vs. MMF (A and C) and MMF vs. MMF + greca (B and D) comparisons.

Comparing PMF vs. MMF (A and C) and MMF vs. MMF + greca (B and D) by (i) the ratio of netting walls with some catch; (ii) the revenue (in Euros) of the commercialized catches (A and B) or the weight of the discards (C and D) per netting wall with some catch; and (iii) the revenue (in Euros) of the commercialized catches (A and B) or the weight of the discards (C and D) for an average (22 netting walls) net. Only items attributable to a given netting wall were used in these comparisons.

Figure 5. PCA of the discarded items attributable to a given netting wall (points in the figure).

The PMF and MMF largely overlap, but MMF + greca significantly differs in species-specific abundance. This difference is mainly related to a higher abundance of S. canicula.

Figure 6. Abundance distribution (kilogram per netting wall) of the species that contributed most to the differences in the discarded fraction between the different types of manufactured netting walls.

(A) S. canicula; (B) Raja species.

Figure 7. The proportion of discarded animals that were alive when they arrived on board.

The Bayesian 95% credibility intervals are indicated.

Figure 8. Changes in vitality status over time for the spiny lobsters caught and held in captivity for up to seven days.