Prawn sale commission

GST registration is mandatory for-

• Any business whose turnover in a financial year exceeds Rs 20 lakhs (Rs 10 lakhs for North Eastern and hill states).

[Note: If your turnover is supply of only exempted  goods/services which are exempt under GST, this clause does not apply.]

• Every person who is registered under an earlier law (i.e., Excise, VAT, Service Tax etc.)  needs to register under GST, too.
• When a business which is registered has been transferred to someone/demerged, the transferee shall take registration with effect from the date of transfer.
• Anyone who drives inter-state supply of goods
• Casual taxable person (see below)
• Non-Resident taxable person (see below)
• Agents of a supplier
• Those paying tax under the reverse charge mechanism
• Input service distributor (see below)
• E-commerce operator or aggregator
• Person who supplies via e-commerce aggregator
• Person supplying online information and database access or retrieval services from a place outside India to a person in India, other than a registered taxable person

As for Exempted goods/services no, commission on sale of prawns is taxable and not Exempt.

Thus yes unless you are not covered in any of the above conditions you will be required to register under the GST Regime.