Inflation is a measure of the relative purchasing power of a unit of currency—in this case, a dollar. Inflation is a term typically used to denote an overall trend, as opposed to a change in a specific individual's purchasing power. But let's consider the individual perspective for a moment. Based upon the information given, we cannot conclude that the sellers of fish, meat, or fruit are better off, or worse off, than before, because we lack information about how great the supplies of those commodities are, what proportion of the market they each constitute, and what the consumer responses are to the change in prices. If each of the commodities constitute 1/3 of the market and when the price of fish doubles, everyone eats only meat and fruit instead of fish, then, overall, the consumers are spending less money and experience an increase in their buying power, which would not constitute an overall weakening of their buying power, or "inflation."
But if for some reason fish was a required (and majority) part of the population's diet, then they'd be spending twice as much, overall, for the same amount of fish or the same amount for half as much fish. Individuals are free to alter their behavior in response to price changes, so some individuals could choose to purchase meat instead of fish and that would save them money yet perhaps disadvantage the meat producers. But because we don't know what percentage of the market each food group accounts for, or what percentage of the population consumes how much of each, or what proportion of the population work in each of the individual industries, it is impossible to tell whether or not there is an overall increase or decrease in the purchasing power of the dollar.
In order to construct a simple measure of the "change in the [overall] price level", you would need to construct an equation consisting of the proportion of the market each commodity represents and their respective prices at the beginning of the year to come up with the "average" commodity price. Call this "P1". Then for the end of the year "average" commodity price comparison, you would need to update the figures in your equation for not only the "new" prices, but also for the "new" proportions of the market that each commodity represents to take into account any changes in purchasing behaviors during the year. Call this "P2". If P2 is larger than P1, then it is fair to conclude that inflation has occurred.