The rules of a fishing game

We played a game today.  We split into three groups, each of which ran a fishing company on a stretch of coast with a healthy population of fish.  The fish multiplied at a regular rate for fish--if there was overcrowding (which we assumed at the beginning there would be), they would reproduce at a lower rate, just as if there were few fish, fewer would be born.  But if there were about 60% of the maximum number of fish, they would reproduce at the greatest rate.  We each got our little fleet of ships, with opportunities to buy more to increase our profits (umm...wrong).  By the third turn (year), we had more than enough ships to run the place dry, and two teams had merged to compete with the third.  But, even with collusion the next year, we depleted both the deep sea fisheries where there were greater profits and the coastal ones with less profitability.
Anyway, it was amazing how fast we depleted the fish.  We/I did the math wrong, so we were overfishing from the start, and once the fish catch started to decrease, the fish were already at 30% of maximum--half the optimal level.  At that point, we would have had to wait 4 years or more of no fishing (of only 10 years that the game went for) until it would have been back at 60%, producing the most fish per year.  The problem is, we weren't thinking like it was a farm, but like it was a competition.  We were scared of not making any money.  So we kept fishing.  And at the end of ten years, we just fished all the rest of them anyway.
We began with 8 ships per team, with the rule that we could only purchase half again of what we own in a turn.  It turns out if we had read the graph right, you need 22 ships for the deep sea, and 20 for the coastal region.  That's 42 ships--divided equally, that would be 14 per team.  So, that team that won the first auction did not need any more ships.  They ended up with 52 ships (which you have to pay every turn to upkeep--fishing in coastal waters is three times as expensive as leaving them in harbor, and deep sea fishing is five times expensive as harboring), which was a huge liability.  Our team only stayed afloat. (hee hee) by merging with the other team that had few ships.  Although we ended up with 49 (6 of which we got for free, but I'm pretty sure we lost money on them).  It seemed like a "buy more ships, make more money" situation.
If we had maximized early, more than $40,000 worth of cash would have flowed into the game over its course from reproducing fishies, not to mention our fleet of boats would have been worth about $6800 (divide each of those by three!).  We ended up with one company in the red and one barely over $10,000.  The coastal species of fish was extinct in the region, and the deep sea population was only at 11% of maximum--about 300 fish were left.
Anyway, super frustrating.  The principle is this--as soon as you start noticing a decline in fish population, it's too late.  You've already reduced the fish to extremely low numbers.  The only way to have fish reproducing (the only long-term capital input into your business) is to harvest only as many as the fish can replace.  Otherwise you're digging in to your "principal," as if this were a bank account supporting you.  That's probably why they called it "FishBank."  Ugh.  We were overfishing after 3 turns.