As
@McQuade noted, it would be very difficult for Tassal to buy out their sole competition because of the ACCC, and no doubt Coles and Woolies would be having conniptions. The ACCC will define it as salmon because it's a large enough market, and wouldn't consider import as a sufficient competitive force. So any local acquisition of Huon would need to be another diversified food company I reckon, and FIRB would likely stop international acquisitions because of all the environmental / water rights locked up in it.
The other issue is that even if TGR did do a capital raising to fund an acquisition, they are not "selling" the shares to the markets to fund it. They are creating new equity via their acquisition, so in theory it would not dilute the shares and drive the price down*. If you add $100m of equity by creating $100m of shares, it's not driving the share price down. If the price does go down, it's because you are paying $100m for only say $50m of equity as per the valuation by the market. So in effect that over-valued price you are paying is actually a reflection of Huon being worth less than the price Tassals would be paying in your scenario.
*There could be a point to make that there would be insufficient institutional interest for TGR's capital raisings, and they would need to offer a really good price to get the funds they need. I think considering how beaten the stock price is, there is an argument to be made here. Bega did alright recently though, as did Select Harvest. And private equity is often quite interested in these areas. So I am not sure this would be sufficient for a 13% short at least..
