As you probably know, Denise, smart Georgina dinners were usually of two or three courses( two courses plus dessert).One course dinners were the norm when the family was not very grand or had no visitors.

Here is an extract from one of Jane Austen’s letters to Cassandra where she describes a typical one course family dinner , which was served to her at her brother Henry's house in London, in 1813:

We arrived at a quarter-past four, and were kindly welcomed by the coachman, and then by his master, and then by William, and then by Mrs. Pengird, who all met us before we reached the foot of the stairs. Mde. Bigion was below dressing us a most comfortable dinner of soup, fish, bouillée, partridges, and an apple tart, which we sat down to soon after five, after cleaning and dressing ourselves and feeling that we were most commodiously disposed of.
Written from Henrietta St., Wednesday ,Sept. 15, 1813, 1/2 past 8.

I should perhaps explain that the courses in a Georgian meal differed from our understanding of what consitutes a course. A course in this era was made up of many dishes , mixing sweet and savoury in the same course and all were placed on the dining table at once .The diners would then help themselves to the dishes nearest to them on the table.

Coming to fish, fish dishes were nearly always included in the mix of dishes which constituted a course. They usually were included in the first course of a meal as one of the “removes”. There were usually one or two “removes”: that is, hot dishes such as soup or fish which when eaten would be removed from the table and replaced with another dish. Mrs Bennet would most certainly want to include at least one fish dish in a two course meal in order to impress her guest ;-)

In The Complete Housekeeper by Mary Smith( 1803), Mrs Smith, formerly housekeeper to Lord Anson of Shugborough Park in Staffordshire gave this as an example of a two course meal to be served in November:

In the First Course you can see that the Lentil Soup is to be removed and replaced with Turbot(YUMMY), and a Sturgeon appears in the second course.

Most of the cookery books of this period give marketing directions to cooks , explaining how to spot a bad piece of fish. Some of the books differed in what they thought ought to be in season.

For example, Mrs Smith ,as above, recommended the following fish to be in season in November:

Dorees
Smelts
Barbets
Gurnetts
Carp
Pike
Tench
Whitings
Haddocks
Codlins
Oysters
Cockles
Shrimps
Soles
Turbot
Lobsters.

(page 375)

Whereas Francis Collingwood and John Woollams in The Universal Cook and City and Country housekeeper(1792) advise that

Gurnets
Dorees
Holoberts
Bearbet
Slamon
Salmon Trout
Smelts
Carp
Pike
Tench
Gudgeons
Lobsters
Oysters
Cockles
Muscles( sic)(Page 442)
are all in season.

Mrs Bennet had not been given much notice by her loving husband of the impending visit, so I can understand her frustration in Chapter 13.

However, here is an old post of mine which explains why there was not a bit of fish to be had in inland Meryton: it was most probably a Monday when Mr Collins arrived at Longbourn and as such, no fish would be on sale in Meryton( or indeed anywhere where fish had to be transported form a port).