I know you say you don't like this word here, as it probably makes you think of capital contribution for corporate contexts, it is never the less often used to describe the contribution the fishing industry has made to the economy. I would use it here. "Landings volume" is a term used to describe the quantities caught and actually brought ashore but the word used needs to convey the idea of a contribution. I see no need to use anything else. Also you may need synonyms or close synonyms elsewhere in the text. When the "mot juste" exists, why shy away from it?

http://www.releases.gov.nl.ca/releases/1999/fishaq/1123n02.h...

"Contribution of the fishing industry...

...It gives me pleasure to inform my honourable colleagues of the continued good news involving our fishing industry. I think we all agree that the fishing industry contributes significantly to the economy of Newfoundland and Labrador and, without a doubt, remains the economic cornerstone of our rural economy."



http://www.dfw.state.or.us/fish/commercial/docs/OR_Comm_Fish...

"Introduction
1,2,3
Preliminary economic contribution estimates show 2011 was an average year for some Oregon commercial fisheries and an exceptional year for others. While overall it was an excellent year, some harvesters and processors dependent on certain fisheries had disruptions in their business opportunities. Further, the Year 2011 improved economic contributions were not evenly
distributed among all coastal ports. Trends in vessel industrialization and processor consolidation continued 2011.
4"

"This report is a short accounting about onshore landings volume, harvest value, processor production, participant characteristics, and fishing industry economic contribution estimates in
2011. 6 Economic contributions from distant water fisheries are mentioned. An outlook for 2012 and description of industry challenges is also provided."

"(2) The second highest contributing fishery harvest value is usually the non-whiting groundfish fishery, which at $28.2 million in 2011 continues to hold the ranking."

"Economic Contribution Estimates
The fishing industry generated $284 million total personal income in 2011 due to onshore landings (Table 7). This is compares to a 2010 economic contribution of $228 million. Year 2011 was a 16.3 percent higher economic contribution than the previous five year average of $244 million. This includes the income generated by the spending from harvesting and primary processing sectors. The economic contributions also include the income generated by the expenditures from supporting industries and businesses in the region as well as re-spending by households who receive earnings from the fishing and supporting industry (or the so-called
multiplier effect).
11"

--------------------------------------------------
Note added at 8 hrs (2012-12-15 10:30:13 GMT)
--------------------------------------------------

In order to justify a close translation, and thus to stick with "contribution", if you take into account the fact that the "apport" is being analysed. Further upstream in your original, you may have an indication of the precise nature of the contribution being analysed : value, volume, tonnage etc. Indeed, the references in the sentence afterwards are in terms of tonnage, rather than volume which would be expressed in cubic units, unless you wish to allow for a loose usage of the term "volume", for example. In the absence of such precision, stay close to the original, no?

--------------------------------------------------
Note added at 8 hrs (2012-12-15 10:30:50 GMT)
--------------------------------------------------

(Scrap the "if" in the first sentence please!)