Resumen de The Influence of environmental factors on small pelagic target species (Sardine, Anchovy, and Mackerel) in the southern Alboran Sea (Western Mediterranean)
Small pelagic fishery is the main activity in the Moroccan Mediterranean Sea. This activity has decreased over the past two decades, leading to increased interactions between the top predators and purse seine vessels in the region. Bottlenose dolphin groups attacks fishing boats while circling pelagic fish such as sardines and anchovies, causing huge economic losses to fishermen and putting these protected species at high risk of injuries. While the decline in these small pelagic fisheries is thought to be primarily due to fishing, climate change and its impact on the disruption of the marine ecosystem may also have affected these species, especially since the Mediterranean Sea is a semi-enclosed and highly sensitive to environmental changes. In fact, the reasons for this decline have been largely debated by Moroccan fishermen, who claims that their activities are not the main cause of this decline, and that a great deal of research has been done on the evolution of these fish stocks, as well the fishing impacts (monitored annually by the INRH), but very few studies have attempted to understand the relationship between the dynamics of these species and changing oceanographic conditions in the region.
In our study, data on the monthly main small pelagic landings (Sardine, Anchovy, and Mackerel) on the Mediterranean Moroccan coast from 1983 to 2020 were obtained from the Moroccan National Office of Fisheries for the main small pelagic fishing ports, Mdiq, Hoceima, Nador, and Ras Kebdana, although some other, smaller ports also contribute to the total sardine landings in this area. The fleet effort (fishing days) was only available from 2009 to 2020. Landings per Unit Effort (LPUE) for this period were estimated as landings in metric tons divided by the number of fishing days.
In addition to small pelagic landings, information on environmental variables that potentially impact sardine recruitment and abundance were obtained from various sources. These variables are sea surface temperature (SST), sea surface salinity (SSS), surface chlorophyll-a concentration (Chl-a), the west-east (u) and south-north (v) components of the wind, the North Atlantic Oscillation (NAO) index, the Mediterranean Oscillation Index (MOI), and the Western Mediterranean Oscillation index (WeMO). SST, SSS, and Chl-a data were averaged over the southern part of the Alboran Sea, from 5.5°W to 2°W and from 35°N to 36°N. The Atlantic Jets flowing in the Gibraltar Strait was another environmental variable with the potential to impact small pelagic, and monthly sea level time series for Algeciras and Ceuta was used as an indicator of the changes in the inflowing velocity.
The linear model reflects the possible effects of warming in the Alboran Sea (possibly related to climate change) and reduced AJ kinetic energy on the observed trend of decreasing small pelagic and sardine landings. Anticipated climate change impacts include increased water column stratification and reduced winter mixing, thereby reducing nutrient injection into the photic zone and reducing the intensity of winter/spring phytoplankton blooms. So far, these changes have not been observed in the Alboran Sea, and the chlorophyll time series show a positive trend. One possible explanation for this is that the effect of ocean warming on the stratification of the Mediterranean water column may be offset by an opposite effect associated with increased salinity. Another possibility is that the Alboran Sea is influenced by specific upwelling mechanisms. Chlorophyll concentrations in surface waters appear to be an important factor regulating the variability in annual sardine landings. Finally, for long-term and inter-annual variation, the linear model was able to explain approximately 60% of the variance in landing.
On the other hand, when modeling the time trend data, it was found that anchovy landings varied significantly. SST, SSS and U-wind are the main environmental factors affecting the abundance of anchovies. However, environmental factors could not explain the variation in anchovy landings after removal of the trend component. This may indicate that environmental factors, while having long-term effects, have little effect on inter-annual timescales.
As for the possible effects of environmental changes on Atlantic chub mackerel. The results suggest that this specie may be more sensitive to salinity, temperature, ocean current and wind patterns, and anchovy (prey) abundance, and that the evolution changes in these variables may affect their abundance and ultimately fishing exploitation. On the other hand, the increase of temperature and salinity could possibly favorite the expansion of this specie on behalf of other small pelagic studied species (Anchovy, and Sardine). These results demonstrate the need to consider the impact of oceanographic processes when managing these fishery resources and suggest that the dynamics of these pelagic fish populations in the Alboran Sea are significantly influenced by the inflowing Atlantic currents. It also notes how difficult it is to predict how climate change will affect small pelagic species due to the lack of established long-term monitoring plans, scarce data and complex environmental changes. The impact of environmental change on ecosystem functioning in general, and on small pelagic species in particular, requires further study in the future.
