Question: How many squid should I dissect to determine the average amount of micro plastics in squid guts?

This paper has some solid evaluation of the different ways to digest the squid guts: https://www.sciencedirect.com/science/article/pii/S0025326X19301845

This paper looked at 64 individual anchovies from local waters: https://www.researchgate.net/publication/308813073_Microplastic_fragments_and_microbeads_in_digestive_tracts_of_planktivorous_fish_from_urban_coastal_waters

I think that you can pick any number of fish to fit the amount of time and materials you can allocate to the project. If you would like to choose a number that has statistical significance, this thread has some information and resources on how to select the sample size for an experiment involving animal subjects that you could adapt: https://www.researchgate.net/post/How_can_I_calculate_the_sample_size_in_an_animal_trial

@mimiss has marked @amallozzi as a co-author.

Here’s a link to the chapter on determining sample size in an ecological study from Charles Krebs’ Ecological Methodologies textbook: https://www.zoology.ubc.ca/~krebs/downloads/krebs_chapter_07_2017.pdf

He walks the reader through a couple of standard options for how to figure this out based on the requirements you have for your results. He also provides an option near the end of the chapter for if none of the standard ways work for your study.

There are also sample size calculators available on the internet, but I think the problem you might run into with those is knowing how big your population size should be.