Assessing fishing and marine biodiversity changes using fishers' perceptions: the Spanish Mediterranean and Gulf of Cadiz case study

Abstract

Background: The expansion of fishing activities has intensively transformed marine ecosystems worldwide. However, available time series do not frequently cover historical periods.

Methodology: Fishers' perceptions were used to complement data and characterise changes in fishing activity and exploited ecosystems in the Spanish Mediterranean Sea and Gulf of Cadiz. Fishers' interviews were conducted in 27 fishing harbours of the area, and included 64 fishers from ages between 20 to >70 years old to capture the experiences and memories of various generations. Results are discussed in comparison with available independent information using stock assessments and international convention lists.

Principal findings: According to fishers, fishing activity substantially evolved in the area with time, expanding towards deeper grounds and towards areas more distant from the coast. The maximum amount of catch ever caught and the weight of the largest species ever captured inversely declined with time. Fishers (70%) cited specific fishing grounds where depletion occurred. They documented ecological changes of marine biodiversity during the last half of the century: 94% reported the decline of commercially important fish and invertebrates and 61% listed species that could have been extirpated, with frequent mentions to cartilaginous fish. Declines and extirpations were in line with available quantitative evaluations from stock assessments and international conventions, and were likely linked to fishing impacts. Conversely, half of interviewed fishers claimed that several species had proliferated, such as cephalopods, jellyfish, and small-sized fish. These changes were likely related to trophic cascades due to fishing and due to climate change effects. The species composition of depletions, local extinctions and proliferations showed differences by region suggesting that regional dynamics are important when analysing biodiversity changes.

Conclusions/significance: Using fishers' perceptions, fishing and ecological changes in the study area were documented. The recovery of local ecological knowledge provides valuable information complementing quantitative monitoring and evaluation surveys.

Figure 1. Study area in the Spanish Mediterranean Sea and Gulf of Cadiz.

Harbours where interviews were conducted are indicated.

Figure 2. Proportion of a) fishers interviewed by age range (n = 64), and b) fishing methods used by fishers.

In Figure 2b fishers can be represented more than once due to results integrate their knowledge through their entire working life, thus the total proportions do not sum to 1.

Figure 3. Mean and standard deviation of a) depth (m) and b) distance from the coast (km) between past and present fishing activities.

Figure 4. Spatial representation of mean depth and range values between past (mean  = 100, range  = 1–600 m) and present (mean  = 200, range  = 1–1000 m) fishing activities.

Figure 5. Mean and standard deviation of a) depth (m) and d) distance from the coast (km) between past and present fishing activities by age group (fishers younger or older than 40 years old).

Figure 6. Assessment of the a) best catch in a day of fishing (t) and year of starting fishing (n = 47), b) best catch in a day of fishing (t) and year of starting fishing, excluding small pelagic fish and squids (n = 43), and c) best catch in a day of fishing (t) (n = 43) and year of the best catch.

(Species drawings: www.gencat.cat).

Figure 7. Assessment of the a) largest caught species in a day of fishing (t) and year of starting fishing (n = 58), and b) largest caught species in a day of fishing (t) and year of starting fishing, excluding whale and basking sharks (n = 56).

(Species drawings: http://ian.umces.edu/symbols).

Figure 8. Composition of the largest caught species listed by fishers a) by decade and main species group, and b) main cartilaginous and bony fish species reported during the first and last decades and their mean length (species drawings:http://ian.umces.edu/symbols & www.gencat.cat).

Figure 9. Largest common hake individuals caught (kg) by a) years of starting fishing (n = 30) and b) year of the largest caught individual (n = 25).

(Species drawings: www.gencat.cat).

Figure 10. Assessment of the a) year of the best catch and of the largest caught species, and b) year at starting fishing and the largest caught species.

Diagonal grey line represents the x = y function.

Figure 11. Temporal evolution of a) Bottom trawling fishing effort in the Blanes harbour (NW Mediterranean Sea) from 1900 to 2013 (HP; data sources: Annex S1), b) Bottom trawling fishing effort in the Balearic Islands (NW Mediterranean Sea) from 1920 to 2010 (HP; data sources: Annex S2), c) Ebro River water runoff from 1953 to 2010 (m3/s, annual mean; data sources: Ebro Hydrographical Confederation, Tortosa station,http://www.chebro.es/) and Sea Surface Temperature (SST) from 1970 to 2007 (°C, annual mean; data sources: data was collected near Tarragona, 40°N–2°E, Smith and Reynolds, 2004); and d) Western Mediterranean Oscillation Index from 1950 to 2009 (WEMOI; Error! Hyperlink reference not valid., [76]).