In the Ensenada de La PazBaja California Sur, fishing for ax callus (Atrina maura) is facing a silent but devastating threat: the invasive tunicate Distalpia styliferaa species from the Red Sea that has completely altered the equilibrium of the marine ecosystem.

Despite the impact, members of the Fishermen's Organization Rescuing the Inlet (OPRE) have managed to keep their fishery alive thanks to an exemplary practice of sustainable management of this resource.

With the collaboration of OPRE y NOS Sustainable Northwestthe Bioeconomy Group of the Northwest Biological Research Center (Cibnor), headed by the Dr. Fernando Aranceta Garzaconducted a study on the effects of an exotic tunicate on the callo de hacha fishery in Ensenada de La Paz. and the most important finding is that this invasive species is slowing the growth of the bivalve mollusk population.

"During the restoration of the callus in the Ensenada de la Paz, any type of fishing was prohibited, the population of hatchet callus increased considerably thanks to the restoration effort of the fishermen., and yet there was a very sharp drop in 2016 caused by the invasion of this exotic species of tunicate, and to date the population has not grown as expected (up to 7 individuals per square meter), it remained stagnant at 0.2 individuals per square meter," he explained.

The researcher indicated that by extrapolating the density with the area reported for the callus banks of the Ensenada de La PazThe harvest quota is defined each year by the authority, which applies an approximate estimate of how many hatchet corns there are. Each year, the authority defines the extraction quota, applying the precautionary approachThe percentage of organisms that have already reached harvest size, allocating between 15 and 20% of the percentage of organisms that have already reached harvest size, as the official quota.

A problem with serious effects

The tunicate acts as a parasite of the ax callus. It attaches itself to the exposed shells of hatchet callus, covers them completely and causes their death. This phenomenon has had serious consequences: at the opening of the fishing season in 2017, they managed to produce only three tons, and in recent times, the catch has fallen to less than one ton.

Aranceta Garza stressed that based on the study they now know that approximately 60% of the mortality of the callus population at the Ensenada de La Paz is directly attributed to tunicate, compared to 30% due to natural causes and only 10% due to fishing.

"If there were no tunicate, what we found is that the restoration effort would probably have reached the maximum population of the ecosystem, which is more or less 7 callus per square meter over a period of 6 years. The incredible thing is that they are currently operating with 0.2 callus per square meter, and due to the high price that callus fetches, this has been enough to continue operating," he commented.

Sustainable management, the key to conserving the scallops

In view of the presence of an invasive species that does not allow the population growth of the ax callusthe sustainable management that for years have been carried out by the fishermen of the OPRE is what has made it possible to conserve this fishery resource in the Ensenada de La Pazsaid Aranceta Garza,

"They were always working in a sustainable way, apart from that they apply a precautionary approach, where they are still extra careful with the way they extract the resources. That's what allowed them to continue fishing, even when the tunicate wiped out 80% of the population. That is the incredible thing. In other circumstances, if there were illegal fishing, or if they had not respected the catch sizes, or the application of quotas based on their estimated biomass per year, the resource would surely have collapsed", he pointed out.

The researcher considered that this is a national example of how a fishery managed by fishermen with their monitoring, quota estimation, applying a precautionary approach and self-regulating, can be sustainable, despite suffering external impacts.

Adaptation and resilience to the new reality

As in the tunicate favor warm waters and in view of the scenario of the climate change This could complicate the current situation, which in itself, this type of invasive events, once installed, is very difficult to eradicate, it is even almost impossible, said Dr. in Fisheries and Aquaculture Bioeconomy.

In view of this new tunicate regime in the Ensenada de La Pazfishermen should be aware that they will not be able to reach the fishing levels of the ax callus as before and have to consider the effect of this invasive species on the management of the fishery, to develop strategies to overcome it, he said.

"We have to do feasibility studies where we can explore fishing strategies that consider the tunicate and ways to mitigate its effect and see which one works. The goal is to lower the mortality caused by the tunicate," he said.

"I would tell them (fishermen), 'this is like a pressure cooker. The resource cannot leave or grow because of a barrier (the tunicate). The moment you remove the barrier, population exploitation is going to occur'. When we say exploit is that, if we manage to reduce mortality by the tunicate, it is very likely that, given its biological characteristics, the callus will recover, because it is a fast-growing species and in less than a year and a half there are already reproductive organisms generating recruits to the same population. So any decrease in mortality will reflect positively on the population and that will be reflected in the catch, which is what they want.

Possible solutions

As the tunicate is capable of adhering to artificial surfaces, in order to prevent its proliferation, Aranceta Garza proposed the activation of regular cleaning campaigns in the fishing areas, in addition to aontrol of axe callus densities y studies of tunicate growth dynamics to assess how to mitigate its effect.

However, these efforts require financial support because they are costly and with the current income fishermen would not be able to afford the necessary actions to stop this invasion.

"The biggest challenge is to reduce the effect of mortality caused by the tunicate, to stop this invasion or this effect, but it requires a great deal of logistics and sources of financing. It is necessary to look for national or international funds, which support questions of fishing sustainability or natural resources", he emphasized.

The first step is to implement a strategy to reduce the effects of the tunicate in the ax callusHe insisted that the fishermen could achieve self-sufficiency and greater socioeconomic well-being, but this is the time when they need help.

The researcher emphasized that, as demonstrated by the fishermen from OPRE with the scallops, it is not enough just to have good fishing practices (fishing in a sustainable way), it is necessary to adapt to the new realities and incorporate the "new player (tunicate)" in the management strategies, because this can make the difference between the collapse and the recovery of a species.