Wednesday, May 28, 2014

Fish eggs and larva

The declining state of fisheries in many parts of the world, combined with the call for more fisheries-independent monitoring, highlights the need for novel ways of identifying and assessing the dynamics of fish populations. Fish life cycles often involve development stages with different habitat needs, which are reconciled by adult movement to spawning grounds, followed by egg and larval dispersal, and then migration to habitats used by pre-adult and adult stages.

Studies on fish eggs and larva are usually limited by the ability to properly identify species. However, this information can be important especially when it comes to the design of marine protected areas. The presence of eggs at particular locations can reveal spawning areas. Egg and larval distribution can also provide us with information on dispersal mechanisms and how (and how deep) habitats are connected.

DNA-based identification systems such as DNA Barcoding can provide a lot more detailed information, in many cases they are probably the only method that will give a reliable answer. For example earlier stock estimates based on egg surveys for Irish Sea cod assumed that the majority of 'cod-like' eggs were actually from cod. When this was tested with a DNA-based method years later it turned out that only 37% of those eggs originated from cod.

There is no doubt that DNA Barcoding can help a great deal with such surveys. About 15 000 fish species have been barcoded so far (Number taken from BOLD's taxonomy browser) which provides a good basis for any egg or larval identification attempt. In a new study a group of researchers from the University of South Florida used DNA Barcoding to compare the community compositions of planktonic fish eggs and larvae within a coastal embayment, testing the hypothesis that the composition of the planktonic larval fish community proportionately reflects the composition of the planktonic fish egg community (excluding species with non-planktonic eggs).

The study revealed an unexpected high degree of spatial heterogeneity in fish egg compositions within the sampled area. Species that dominated the egg community (Eugerres plumieri, Cynoscion nebulosus, Centropomus undecimalis, and Prionotus spp.) were not particularly abundant amongst the 276 larvae identified, and the most abundant larval species (Achirus lineatus and Cynoscion arenarius) only comprised a minor proportion of the identified eggs.

The authors conclude that the clear disparities observed between the species compositions of the egg and larvae highlight the need for directly identifying eggs when studying habitat connectivity or performing stock assessment with egg production model-based methods.

Thismeans that there is actually no reasonable alternative to DNA Barcoding as it is the only method around that provides the necessary level of accuracy and reliability. 

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