It is preferred to have more than 20 length classes (up to 25) for efficient results using Fisat. Consult Gayanilo and Pauly (2005) paper.

Tried that too but in that case the fishing mortality comes to be negative

That can be negative, you must have seen the catch of forage fish in the lake. I think it is very minimal compared to their population.

One potential method for labeling rat fetuses in utero is through the use of fluorescent dyes. Specifically, a vital fluorescent dye called carboxyfluorescein diacetate succinimidyl ester (CFSE) can be used to label cells in vivo. This dye is taken up by cells and becomes covalently linked to intracellular proteins, allowing for long-term labeling of the cells.

To label the rat fetuses, the CFSE dye can be injected into the uterus through the maternal abdominal wall using a fine-gauge needle. The dye will diffuse into the amniotic fluid, allowing it to be taken up by the developing fetuses. The fetuses can be monitored using ultrasound to ensure that they are properly labeled.

After the fetuses are born, the CFSE-labeled cells can be visualized using fluorescence microscopy or flow cytometry. The treated and untreated fetuses can be distinguished based on the presence or absence of the fluorescent label.

It's important to note that the use of CFSE dye may have potential effects on the health and development of the fetuses, and the injection procedure carries risks to both the fetuses and the mother. Therefore, it is important to consult with an experienced veterinarian and follow proper safety protocols when performing in utero labeling of rat fetuses.

You can use TropfishR package rather than FiSAT.

I have located a web link to a research article that suggests an ideal bin size for ELEFAN analysis in stock assessment research. The article's abstract provides guidelines for selecting the optimal bin size.

The link to the article is available at:

https://www.sciencedirect.com/science/article/abs/pii/S0165783619303297

Published in the journal Fisheries Research in 2020, the article outlines a comprehensive methodology for identifying the most suitable bin size for ELEFAN analysis, which is a prevalent method for assessing size-frequency data in stock assessment.