There is currently very little information on the survival of discards of unwanted and unregulated catches of invertebrates after the stresses caused by capture. A great number of the unregulated invertebrate species form the basis of essential fish habitats for important fisheries resources such as hake, red mullet and cuttlefish. Thus, data on their survival after discarding may help to interpret the role of these species within the benthic ecosystems. Furthermore, descriptor 6 of the Marine Strategy Framework Directive (EU Directive 2008/56/E) foresees maintaining sea floor integrity at a level that ensures that the structure and functions of the ecosystems are safeguarded, and Article 7(d) of the Common Fisheries Policy (EU Reg. 1380/2013) foresees the implementation of management measures for fishing with low impact on the marine ecosystem and fishery resources. Survival measurements by direct recovery of tagged discarded species are not effective in bottom trawl fisheries, for which alternative studies such as semi-quantitative measures obtained on board prior to discarding can be considered as appropriate for mortality estimation. The present work assessed the survival of unwanted species using a semi-quantitative assessment on the deck of trawlers and at the laboratory for a period of 96 hours in two Mediterranean areas (the Catalan coast and the Ligurian and Northern Tyrrhenian seas). A high number of discarded invertebrates showed a high percentage of survival (>70%) in both assessments. The results can be used to provide information that can help to achieve higher survival levels of discarded specimens and enhance the productivity of fishing grounds by increasing the health of benthic ecosystems.
Actualmente, hay muy poca información sobre la supervivencia del descarte de los invertebrados no-deseados y no-regulados después del stress de la captura. Un gran número de especies de estos invertebrados son básicas para los hábitats esenciales de importantes recursos pesqueros como la merluza, el salmonete o la sepia. Por lo tanto, información sobre su supervivencia al ser descartados pueden ayudar a interpretar el papel de estas especies dentro de los ecosistemas bentónicos. Además, el punto 6 de la Directiva Marco de Estrategia Marina (MSFD, EU Directive 2008/56/E) prevé mantener la integridad del fondo marino a un nivel que garantice la protección de la estructura y de las funciones de los ecosistemas, y el artículo 7 (d) de la Política Pesquera Común (PPC EU Reg. 1380/2013) prevé la implementación de medidas de gestión para la pesca, que tengan bajo impacto tanto en el ecosistema marino como en los recursos pesqueros. En el caso de las pescas de arrastre de fondo, no es efectiva la estimación de la supervivencia a partir del método directo de captura-recapture de las especies del descarte marcadas, por lo que los estudios alternativos, como las medidas semicuantitativas obtenidas a bordo antes del descarte, pueden considerarse una estimación de la mortalidad apropiada. El presente trabajo evaluó la supervivencia del descarte de especies no-deseadas utilizando una Evaluación Semicuantitativa (SQA) mediante dos estudios: uno sobre la cubierta de los arrastreros y el otro en laboratorio durante un período de 96 horas, en dos áreas del Mediterráneo, las costas Catalanas y en los mares de Liguria y del Tirreno Norte. Los resultados mostraron en ambos estudios, que un alto número de invertebrados descartados tenía un alto porcentaje de supervivencia (>70%). El propósito de estos resultados es proporcionar información que puede ayudar a alcanzar niveles de supervivencia más altos de los individuos descartados. Al mismo tiempo, se espera que con esta mejora se consiga potenciar la productividad de los caladeros al aumentar la salud de los ecosistemas bentónicos.
Mediterranean fisheries are characterized by a high rate of unwanted catches and a great number of marine organisms that are discarded at sea (
The investigated areas were the Catalan coast, corresponding to FAO division 37.1.1, Geographical Sub-Area 6 (GSA06), and the Ligurian and northern Tyrrhenian seas, corresponding to FAO division 37.1.3, Geographical Sub-Area 9 (GSA09), both comprising chronically exploited fishing grounds. In the last ten years the demersal fisheries carried out mainly by bottom trawl fleets in the two areas accounted for about 40% of the total landings and 70% of the economic value (
A large fraction of this discarded biomass (30%-50% of the total biomass caught) is composed of species of commercial interest (small-sized or damaged specimens), while the remaining fraction is composed of species with low or no economic value (
The impact of bottom trawling on benthic habitats and communities and demersal species is little known. The impact depends on the fishing activity (
The European Marine Strategy Framework Directive (EU 2008/56/E) encourages member states to move towards an ecosystem-based fishery management in order to protect the goods and services that marine ecosystems provide. Therefore, it is important to take into account the link between benthic communities and habitats and fisheries resources, because a great number of ecological interactions may be adversely impacted by fishing. The capture of benthic invertebrates and their discarding at sea will impact benthic habitats to a certain degree, depending on the post-release survival of each species. The survival of habitat-structuring invertebrates, such as crinoids and echinoderms, can help to maintain the good status of the essential fish habitats where the most important commercial resources, such as European hake, red mullet, spiny lobster and cuttlefish, use them as areas of nursery, recruitment or growth (
There is currently very little information on the survival of unwanted and unregulated invertebrates after the stresses of being captured, handled and discarded. The specific biological characteristics make an organism more or less vulnerable to different stressors of the capture method and release process (
Some unwanted and unregulated invertebrates such as crinoids and ophiuroids form the basis of essential fish habitats for commercial species such as hake and red mullet. Robust information on discard survival after fishing and release to the seabed can improve the interpretation of the role of unregulated invertebrates on the benthos (
The main objective of this paper was to estimate survival rates of invertebrates discarded from trawlers working in two northwestern Mediterranean areas, the Catalan coast and the Ligurian and northern Tyrrhenian seas. The study focused on unwanted invertebrates which belong to the unregulated species and are likely to continue to be released after capture. To estimate the vitality rates, a vitality assessment on the captured organisms was carried out under normal fishing activity of the trawl fleets in both selected areas. The approach was developed using a semi-quantitative assessment (SQA) (
The study was carried out in two NW Mediterranean trawl fishing areas, the Catalan coast (GSA 6), from March 2016 to February 2017, and the Ligurian and northern Tyrrhenian seas (GSA 9), from November 2016 to February 2017. Data were collected during fishing trips on board commercial vessels performed on a monthly basis.
Trawl sampling in the Catalan coast area was performed on board four different trawlers in five fishing grounds (Garotes, Las 40, Capets, Planassa and Malica) adjacent to the port of Blanes. The depth range was between 50 and 494 m, with a total of 23 hauls (
Study area | Fishing ground | No Hauls | Min. Depth (m) | Max. Depth (m) | Target species |
---|---|---|---|---|---|
Catalan coast (GSA06) | Las 40 | 3 | 85 | 120 | Red mullet, monkfish, hake, octopus and sea cucumber |
Capets | 2 | 70 | 113 | Red mullet, monkfish, hake, octopus and sea cucumber | |
Planassa | 6 | 86 | 318 | Red mullet, sea cucumber, hake and monkfish | |
Garotes | 2 | 55 | 107 | Red mullet and pandora | |
Malica | 9 | 195 | 494 | Norway lobster | |
Ligurian and northern Tyrrhenian seas (GSA09) | Argentario | 22 | 85 | 470 | Hake, red mullet, horned octopus and deep- water pink shrimp |
Table S1 of the Supplementary Material details the main characteristics and environmental data of each haul in the two selected areas. In both areas the average haul duration was about two hours.
At the end of each haul the trawl gear was retrieved on board and the cod-end was opened on the deck following the normal commercial fishing practices. After that, prior to sorting the catch into commercial and discard fractions, the net was shot for a new haul. Depending on the depth, this process took 10 to 25 minutes. During the fishing trips, there was no interference by the researchers on board with the habitual modus operandi of the fishermen in the daily fishing activity (position, duration, sorting, etc.) and the sorting processes of the catch, which lasted 20-30 minutes depending on the capture.
The hauls considered for survival analysis were 4 on the Catalan coast and 19 in the Ligurian and northern Tyrrhenian seas (Supplementary material Table S1A). The average depth range was between 99 and 362 m on the Catalan coast and between 84 and 470 m in the Ligurian and northern Tyrrhenian seas. The hauls were carried out on both the continental shelf and the upper slope, with a standard deviation of 117.66 and 152.89 m, respectively. The complexity of the experiment forced us to limit the number of hauls. While the catch was being sorted manually on deck by the fishermen, in both areas the vitality of the unregulated, non-commercial invertebrates was assessed just before the species were discarded. The SQA (
Vitality levels | Code | Crustaceans | Echinoderms (Ophiuroidea and Asteroidea) | Echinoderms (Echinoidea) |
Mollusca | Sessile (ascidians, corals, hydroids, etc.) |
---|---|---|---|---|---|---|
Excellent | 1 | Continued movement; no external injury. | Continued movement; no external injury. | Continued movement; no external injury. | Continued movement; no external injury. | Shape and size similar to natural state. No external injury. |
Good | 2 | Weak movement; responds to contact; no external injury or superficial cuts on the exoskeleton or antennae. | Weak movement; responds to contact; no external injury or superficial cuts in limbs. | Weak movement; responds to contact; no external injury or few broken spines | Weak movement; responds to contact; scraping of shell or moderate loss of tegument. | Size and shape moderately different to natural state; moderate cuts or abrasions. |
Poor | 3 | No apparent movement, but can move antennae or maxillipeds; loss of a member or deep cuts. | No apparent movement, but can move tube feet; deep cuts and loss of all or part of extremities. | No apparent movement, but can move tube feet; external injury and many broken spines. | No apparent movement, but can move feet, loss of parts of the shell or limbs. | Shape and size different to natural state; surface or serious cuts or abrasions; loss of body parts. |
Dying or dead | 4 | No movement; does not respond to contact. | No movement; does not respond to contact; loss of central parts of body. | No movement; does not respond to contact; or broken shell. | No movement; does not respond to contact; or broken shell. | Loss of central parts of the body. |
Two approaches to performing the survival experiments were developed: i) direct survival estimation on deck and ii) survival estimation at the laboratory for a period of 96 hours. In both study areas the studied invertebrates are only captured by trawling. Because of the challenges of obtaining viable control samples with other gears, the individuals who were classified in excellent state of vitality (
In both areas the immediate survival on deck was estimated following the same methodology. For the selected species, as many individuals as possible were taken and one of the four VLs was assigned to each one using the SQA (
The long-term survival rate was estimated for 96 h to achieve a deeper knowledge of the actual survival of the invertebrates discarded. It was only performed in the Catalan coast area. To analyse survival at the laboratory, the individuals of the selected species were sampled from the last daily haul, just when the catch was laid on deck and prior to sorting, but only during the first 30 minutes, as in the previous case. The VL was assessed according to the SQA (
Survival at the laboratory was estimated for 96 hours in an aquarium at the ICM laboratory by applying an SQA. A total of ten time survival observations (T) were carried out during the experiment (from T0 to T9). The first observation, time T0, just as animals were being released on deck, was executed on board. Individuals were selected for a maximum time of 30 minutes, and each one was introduced successively into the containers, as explained above. The second observation, T1 (6 h), was performed just before each individual was transferred to the aquariums at the laboratory. The individuals that were in VL 4 (dead or moribund) were removed to avoid contamination in aquariums, but were accounted for. The following eight observation times, from T2 (18 h) to T9 (96 h), were made on the specimens placed in the aquariums, with a periodicity of 12 h until the study had lasted for 96 h. The aquariums were divided into three sections for each VL, 1, 2 and 3, and no more than ten individuals were introduced per section. The time of the transport from the sea to the aquariums was a maximum of 2 hours and the animals were on the white plastic containers with oxygen pills during the whole transportation time.
The natural environment conditions were simulated in the aquariums through an open seawater circuit and water temperature was maintained between 13°C and 14°C, similar to the in situ temperature in the northwestern Mediterranean fishing grounds. The photoperiod was adapted to the natural luminosity with black canvas to dim the light. Controls of salinity, nitrates, nitrites and silicates were periodically performed. The whole process was carried out under food abstinence conditions.
The survival on deck was estimated by applying the survival index, which was calculated as the ratio between the number of specimens (VN) with a vitality level of 1 to 4 and the total number of discarded individuals (DN) and was expressed as a percentage.
A Wilcoxon test between the exploratory variables recorded (Supplementary material Table S1) and the survival of invertebrates on deck for the first 30 min on board was carried out. The test analyses the relationship of each variable with survival, comparing the data related to live individuals (VLs 1, 2 and 3) with data of dead individuals (VL 4). Finally, if significant differences were observed between the variable and the survival, the group mean was calculated. The Wilcoxon test was implemented in R 3.4.3 (
Because a seasonal sampling was not performed, variables related to seasonality such air and water temperature were not taken into account. We therefore conducted the analysis only with Depth and Haul Time.
To calculate survival rate for each experiment over 96 h, the Kaplan-Meier analysis was used (
The VLs of discarded invertebrates were identified for each individual of each selected species.
Species | VL 1 | VL 2 | VL 3 | VL 4 | Total alive | Total assessed | Survival index | ||
---|---|---|---|---|---|---|---|---|---|
A, Catalan coast | |||||||||
Continental shelf | Crinoid bed | 19 | 2 | 8 | 0 | 29 | 29 | 100 | |
17 | 37 | 86 | 12 | 140 | 152 | 92.11 | |||
0 | 2 | 4 | 0 | 6 | 6 | 100 | |||
4 | 5 | 7 | 5 | 16 | 21 | 76.19 | |||
2 | 3 | 7 | 0 | 12 | 12 | 100 | |||
3 | 0 | 1 | 0 | 4 | 4 | 100 | |||
9 | 3 | 4 | 0 | 16 | 16 | 100 | |||
Muddy | 0 | 0 | 6 | 0 | 6 | 6 | 100 | ||
1 | 1 | 0 | 0 | 2 | 2 | 100 | |||
4 | 0 | 0 | 0 | 4 | 4 | 100 | |||
1 | 0 | 1 | 1 | 2 | 3 | 66.67 | |||
2 | 0 | 0 | 0 | 2 | 2 | 100 | |||
0 | 0 | 38 | 0 | 38 | 38 | 100 | |||
4 | 0 | 2 | 0 | 6 | 6 | 100 | |||
Slope | Muddy | 5 | 7 | 0 | 0 | 12 | 12 | 100 | |
2 | 0 | 0 | 0 | 2 | 2 | 100 | |||
1 | 0 | 1 | 0 | 2 | 2 | 100 | |||
0 | 0 | 4 | 2 | 4 | 6 | 66.67 | |||
72 | 143 | 222 | 354 | 437 | 791 | 55.25 | |||
Total | 147 | 204 | 387 | 375 | 738 | 1113 | 66.3 | ||
B, Ligurian and northern Tyrrhenian seas | |||||||||
Continental shelf | 0 | 0 | 3 | 0 | 3 | 3 | 100 | ||
Galeodea spp. | 0 | 84 | 0 | 0 | 84 | 84 | 100 | ||
7 | 0 | 0 | 236 | 7 | 243 | 2.88 | |||
45 | 60 | 0 | 0 | 105 | 105 | 100 | |||
21 | 20 | 6 | 955 | 47 | 1002 | 4.69 | |||
0 | 3 | 0 | 0 | 3 | 3 | 100 | |||
10 | 4 | 10 | 519 | 24 | 543 | 4.42 | |||
0 | 0 | 2 | 0 | 2 | 2 | 100 | |||
88 | 134 | 187 | 8923 | 409 | 9332 | 4.38 | |||
45 | 103 | 114 | 1863 | 262 | 2125 | 12.33 | |||
Slope | 4 | 10 | 0 | 28 | 14 | 42 | 33.33 | ||
17 | 17 | 6 | 0 | 40 | 40 | 100 | |||
0 | 2 | 2 | 8 | 4 | 12 | 33.33 | |||
22 | 31 | 32 | 187 | 85 | 272 | 31.25 | |||
2 | 0 | 4 | 13 | 6 | 19 | 31.58 | |||
2 | 5 | 25 | 173 | 32 | 205 | 15.61 | |||
5 | 16 | 18 | 452 | 39 | 491 | 7.94 | |||
0 | 0 | 2 | 0 | 2 | 2 | 100 | |||
Total | 268 | 489 | 410 | 13357 | 1167 | 14524 | 46.41 |
A survival index of 100% was shown by 14 species in the Catalan coast area and 6 in the Ligurian and northern Tyrrhenian seas area, and
The percentages of the four vitality levels (VL) of Norway lobster and deep water rose shrimp, the two commercial species subjected to MCRS, are presented in
|
|
|
|
---|---|---|---|
Excellent (1) | 9.10 | 0.98 | 1.84 |
Good (2) | 18.08 | 2.44 | 0.74 |
Poor (3) | 28.07 | 12.20 | 1.84 |
Dying or dead (4) | 44.75 | 84.39 | 95.58 |
The Wilcoxon test was carried out only with the variables Haul Time and Depth. The results showed no significance between depth and survival. However, the test found significant differences in survival due to Haul time with a p-value <2.2e-16 and W=7898500. The mean duration of hauls with live animals was 217.39 min, while the mean duration with dead animals was 236.12 min (i.e. a 9% time increase).
The survival estimation at the laboratory was carried out only for the specimens collected in the Catalan coast study area. The analyses were undertaken only with the invertebrate species with a higher number of individuals scored with VL on deck previous to the discard (
The species
time | n.risk | n.event | survival | sth.err | lower 95% CI | upper 95% CI |
---|---|---|---|---|---|---|
A. Longitudinal survival over 96 hours and three levels of vitality | ||||||
0 | 152 | 12 | 0.921 | 0.0219 | 0.879 | 0.965 |
30 | 140 | 4 | 0.895 | 0.0249 | 0.847 | 0.945 |
42 | 136 | 4 | 0.868 | 0.0274 | 0.816 | 0.924 |
54 | 132 | 9 | 0.809 | 0.0319 | 0.749 | 0.874 |
66 | 123 | 4 | 0.783 | 0.0334 | 0.720 | 0.851 |
78 | 119 | 4 | 0.757 | 0.0348 | 0.691 | 0.828 |
90 | 115 | 5 | 0.724 | 0.0363 | 0.656 | 0.798 |
B. Mortality events at the laboratory at vitality levels 2 and 3 at T0 (0.0H.=3) | ||||||
30 | 86 | 4 | 0.953 | 0.0227 | 0.910 | 0.999 |
42 | 82 | 4 | 0.907 | 0.0313 | 0.848 | 0.970 |
54 | 78 | 9 | 0.802 | 0.0429 | 0.722 | 0.891 |
66 | 69 | 4 | 0.756 | 0.0463 | 0.670 | 0.852 |
78 | 65 | 4 | 0.709 | 0.0490 | 0.620 | 0.812 |
90 | 61 | 5 | 0.651 | 0.0514 | 0.558 | 0.760 |
C. Longitudinal survival over 96 hours at the three vitality levels | ||||||
66 | 38 | 4 | 0.895 | 0.0498 | 0.802 | 0.998 |
78 | 34 | 2 | 0.842 | 0.0592 | 0.734 | 0.966 |
90 | 32 | 2 | 0.789 | 0.0661 | 0.670 | 0.930 |
D. Mortality events at the laboratory at vitality level 3 at T3 (T3.28.5H.=3) | ||||||
66 | 36 | 4 | 0.889 | 0.0524 | 0.792 | 0.998 |
78 | 32 | 2 | 0.833 | 0.0621 | 0.720 | 0.964 |
90 | 30 | 2 | 0.778 | 0.0693 | 0.653 | 0.926 |
E. Longitudinal survival over 96 hours at the three vitality levels | ||||||
6 | 16 | 1 | 0.938 | 0.0605 | 0.826 | 1 |
Similar results were shown for the species
The last species studied in the aquarium was
This study was carried out with those individuals that showed signs of vitality when arriving on board, which means they were still alive. In fact, there was a low number of specimens that could be assessed, and this may indicate the severe impact of trawling on the seabed and benthic communities (
Several studies have evidenced an improvement in the health of exploited resources when effort limitation and seasonal or temporal closures of trawl fishing activities are implemented (
In order to maintain the good status of the sea bottom, one of the priority actions to be taken is to determine the mortality levels of routinely discarded species. A study carried out near the Catalan coast area related the effects of trawl fishing and feeding of the red mullet
The rates of survival shown by invertebrates in both areas investigated in this study showed great variability between VLs of the same species once the individuals had been captured and deposited on the deck of the trawler. Mortality levels also vary from one species to another, depending mainly on the biological and functional traits of each species, such as fragility, emergent or surface position, filter feeding and sedentary motility (
We went one step further in estimating discarded invertebrate mortality by attempting to identify and separate the injuries of each individual on deck according to its VL. Individuals with VL 1 and 2 at time T0 (time of release on deck) survived on deck until they were released into the sea in a maximum time of 30 min, but those with VL 3 showed low survival on deck. The experiments at the laboratory to analyse survival at 96 h confirmed this behaviour for all analysed species. At the laboratory it was evident that when the survival was not 100% it was because the organism was at VL 3 when released on deck, and in fewer cases at VL 2.
The results of the Wilcoxon test indicated that Haul Time was an important factor for improving organisms’ survival on deck. Injuries increased and VL decreased when invertebrates arrive on deck after long hauls, as was observed in the continental shell hauls, which showed a higher survival of species of crinoids and crustaceans (
It must be taken into account that, in addition to the unhealthy state of the invertebrates who died during the experiment, the mortality may also have been due to the captivity conditions, where no food was available and the environment was only similar to the most appropriate habitat. However, the possibility of discarded invertebrates escaping predators or obtaining food is low because of the injuries they suffer during capture (
According to the Common Fisheries Policy and the landing obligation to prevent discarding of regulated species (MCRS, Council Reg. EC 1967/2006, Art. 15.4b), a high level of post-capture survival can be adduced by member states to include an exemption from the landing obligation in their discard management plans. Our results for the survival of
The species selected for survival estimations were the most representative of different taxonomic levels and were of ecological importance in their habitats. In view of this, the 100% survival of the crinoids
The results of the present work offer some new knowledge on the survival of discarded invertebrates that may be useful for improving ecosystem health and productivity. Nevertheless, it should be regarded as a starting point, because mortality after discards at sea depends on many factors, such as susceptibility to predation and lower competitiveness for obtaining food (
This study received funding from the European Commission’s Horizon 2020 Research and Innovation Programme under Grant Agreement No. 634495 for the project “Science, Technology, and Society Initiative to Minimize Unwanted Catches in European Fisheries” (MINOUW). The authors wish to thank José Guzman, Vicens Rovira, Joan Pou, Josep Maria Viñas, Maurizio Bistazzoni and Armando Costaglione, captains of the fishing vessels