Cockels recruiment factors on Testal beach

Testal beach, 27 of march 2017

This section describes an experience developed by a research community made up of scientists from the Campus do Mar and students from the IES Virxe do Mar who jointly carried out an empirical study that involves key research techniques to study the possible global impact on the cockle recruitment ( Cerastoderma edule) in the main shellfish bank of this mollusk of high commercial value in Galicia – Spain.

The investigation focused on the beginning of a temporary series of data that allows to know the possible effects of the global change on the bivalve of commercial interest Cerastoderma edule in the shellfish bank of Testal in Noia (Galicia-Spain). It is the main economic resource of Noia and this shellfish bank is one of the most important in the world in this seafood resource.

The research was proposed as the beginning of a series that they intend to repeat over the next few years, at the same time of year, before the start of the season of the closure, in the last week of shellfish, just before starting the recruitment or passage of larva swimmer to cockle recruit that is located in the sediment.

Research Hypothesis

Cockles shellfish are distributed evenly in beds, regardless of the level of the tide and the size of the animal.

Research Objetive

To test the hypothesis the following objective was proposed:

To study the spatial variability in recruitment and size structure of cockles between tide levels in an intertidal shellfish bank.

Teams and Resposibilities

20 teams of 3 people.

Tasks order:

  • Cockels collection
  • Sieve with water
  • Measurement, counting and writing down of individuals by class and size

Research Methodology

1. Delimitation of transects and sampling points

We spread two tape measures parallel to the coastline, one in the lower mesolitoral and another in the middle one. In each transect, a sampling point was marked with a wooden stake every 25 meters (at least 20 in each transect = 500 meters transect). We stated the initial coordinates of each transect.

2. Definition and extraction of each sampling volume

At each sampling point, with the help of a sampling square, an area of 25×25 cm is marked. With the help of a shovel, sand was collected to a depth of 5cm and was placed in plastic buckets conveniently labeled with pencil in tracing paper labels.

3. Taking sieves to sieve with water

Each bucket was moved to the seashore where we sieved their content through a series of different opening meshes, with the help of seawater. Once sieved, individuals were deposited in trays with their label to be measured with a gauge.

4. Measure with a gauge and notation

For this study we used just the maximum length represented in the image with a L.

These data are written own in a record sheet with a pencil, never with a ballpen.

5. Measuring, counting and classifying

We defined 5 classes per seize from rookies to adults and wrote data down in a chart.

6. Gathering data of each transect for graphic display

We created a shared excel file in Goole drive. Each group introduced data from sampling points in each transect.

With data from excel file we represented the results in a graphic display.

Outcomes graphic display

The graphical representation of the results of collective sampling shows that there are only individuals of three superior size classes. There are just significant differences between the two transects in the adult class, which are more abundant in the low mesolitoral.

Outcomes analysis

There are not spats or small juvenile cockels because it has not yet become recruitment in the sand, as sampling was in late March, when they are just floating larvae.

There’s only a significant difference between the two transects in the adult class, it may be because the sampling was done on the last day of the shellfishing season, so the mesolitoral environment is most affected by the capture. Adults are more easily extracted in the medium mesolitoral than in the lower one because they are longer covered by sea water.

Conclusions

Hypothesis refutation and necessity of a sampling before the beginning of the shellfish campaign.

It is necessary to reformulate the hypothesis that was proved not being true because the position in relation with the tide and the size have influence.

It is convenient a new sampling prior to the start of the shellfishing season in order to verify if the difference in density between the medium and lower mesolitoral enviroment is due to shellfishing or is related with a distribution pattern characteristic of the biology of species.

Pre or post shellfishing season samplings should be done every year to track the effects of climate change such as parasite Martelia Cochillia which invades banks as the water temperature rises.