I made some mistakes & miscalculations in the original question. I have left them that way as it's cleaner to address them in this answer.
Thanks to the input from comments & @TheVagrantDog's answer this is how I think best works for this situation. I'm putting it here in case some other DM may find any of this info useful.
This post is broken into 4 parts: Equipment & Prices, Explanation of how I came to those prices, Fishing Encounter Challenge, Knucklehead Trout Stat-block.
Fishing equipment Special Material
(Prices & explanation listed in section below for ease of reading):
- Hooks: Mithril Hooks (30GP/each)
- Pole: Fishing Rod made of “Elven Yew” (70GP)
- Fishing Line as identified in question [100' length] (52GP)
- Fishing Net, 25 squared feet [optional/highly suggested] (4GP)
- Rowboat [optional/highly suggested] (50GP + 4GP)
The Campaign Guide specifically mentions “Mithril” hooks being available to buy but lists no price for them, it makes no mention of hooks made from Adamantine or other materials. (In the question I originally stated Adamantine because it was stronger than mithril & it was easier to price by comparing a hook to “ammunition”)
Since the book does not give a price we go to the source (DMG) and use some math.
The DMG prices “other items” made of Mithril at +500gp/lb. Great, how much does a fishhook weigh? There are countless sizes/styles of hooks, since these are “big fish” I looked up hooks on the upper end of the scale and on average they weight between 1.5-2oz. For simplicity sake, we'll take 2oz as the base weight and then divide by 1/2 (Mithril items weigh half as normal) leaving us 1oz. Since 16oz=1lb we divide 500GP/16oz to discover Mithril costs 31.25GP/oz. For ease of use, rounding to 1 Mithril hook costs 30GP. (If another DM chooses to say they weigh less that's perfectly acceptable, since the formula is done for you just plug in your new #s)
The Campaign Guide specifically mentions poles made of “Elven Yew” (Yew is a coniferous tree so climate appropriate & they were commonly used to make bows). Again, no price is given nor any guidance to follow like we did for the hooks so we can compare it to what exists and use our best judgement. This could be compared to either a 10' pole or a quarterstaff (both too large with no bend & the 10' pole is too heavy) but a better bet would probably be an “artisan tool” (5GP). Being made of special material I would consider it a “Masterwork tool” (50GP) with an option of adding a special material cost similar to Darkwood (10gp/lb). In the block above, this is what I did. I took the Masterwork Tool, assumed it weighed ½ as much as a Quarterstaff (4lbs) & added 20GP. Still a bit pricey but not too overkill & looks like a good middle ground approach)
Officially, nothing like this exists. The closest item is Silk Rope which is 10GP for 50' but “rope” is too thick for our purposes so I've decided to stick with the “Fishing Line” as identified in the original question. For other DMs who think it's too much, you can consider the line as being part of the fishing pole, I just disagree for this circumstance due to the size & strength of the fish.
As Hey I Can Chan pointed out, a rowboat is only 50GP, (Oars are an additional 2GP/ea). It might be difficult to manage if you land a large catch but you're more likely to catch something in the deeper water than on the shore.
After much “Google-fu” I was unable to find a “fishing encounter” (there are some but not what we're after here) that was equivalent to what we have here. With that, I borrowed some ideas and ended up with this. (Fair warning, depending on your PC's stats & their rolls, this could be a LONG encounter)
PC: Rolls D20 Survival skill check (Alternatively, you could apply a "Profession: Fisherman" check) and apply bonuses you feel appropriate based on masterwork gear.
DM: Secret time check (If PC changes spots before this time is up all new rolls will be required)
1d12: Number of time units
1d3: Determines if time units measured in rounds/minutes/hours
If PC's survival check meets DC set (not identified in answer since I would scale it to PC level) & waits the amount of time rolled by DM he gets a bite
DM: Secret fish size roll 1D8
1-4: Small Knucklehead Trout
5-7: Medium 3HD Knucklehead Trout
8: Medium 4HD Knucklehead Trout
Review Stat block for appropriate HD & go to next table
Player & Fish begin a modified Grapple Check Battle
Players are automatically considered grappled with no attacks-of-opportunity
DM Secret Roll to determine starting distance of fish (1D6×5 feet)
Roll opposed grapple check
The winner gets to move ½ his speed (Pulling or reeling line as appropriate) (Moving a grapple is a standard action) (You may want to consider giving the PC a +1/+2 bonus due to MW pole/line)
- If the Knucklehead Trout wins and the PC does not have a specialized line as described above, the Knucklehead gets a free STR check to break the line
- If the Knucklehead Trout wins and the PC does not have a Mithril Hook as described above, the Knucklehead gets a free STR check to break the hook
- If the Knucklehead Trout wins and the PC does not have an Elven Yew Fishing Rod as described above, the Knucklehead gets a free STR check to break the Fishing Rod
Repeat steps 3 & 4 until either
- Any of the PC's equipment is broken and the fish escapes
- The PC is out of line (go to step #6)
- The Knucklehead is next to the boat (or shore/docks if that's where PC is fishing from) (go to step #7)
If the PC is out of line: Roll opposed Str checks
- If the PC wins, the fish's movement is halted for the rest of it's turn
- If the Knucklehead wins, the player is pulled into the water and is dragged the remaining distance of the Knucklehead's allotted distance (½ his movement), The player may drop his pole to avoid going into the water but the pole will be lost.
- Adjacent players may roll a Ref save to catch the 1st PC, if successful they then roll a STR check to stop the player from going in. If the adjacent player loses the STR check they can either let go of the 1st PC or fall in the water too.
- Players may preemptively tie themselves together or otherwise already be bracing their fellow party members negating the need for a Ref to catch the 1st PC but this carries it's own risks.
If The Knucklehead is next to the boat (or shore/docks if that's where PC is fishing from)
- The PC (or another PC) may throw a fishing net over it & haul it into the boat according to the character's STR score lifting limits (lift off the ground=2X maximum load) (More than 1 person can assist)
Remember, the waters of Icewind Dale are dangerous. These waters are at best just over freezing. Character's who fall in without proper measures will soon suffer from cold effects in DMG. In addition to the cold dangers, at their best conditions the waters are always “choppy” requiring a DC15 swim check (Armor penalties are doubled for swimming!)
Below is the Stat Block I created for the Knucklehead Trout. These numbers were gathered by comparing to the shark & porpoise entries and adjusting to fit. While most likely the adventurers will not be “fighting” a fish, all stats are provided because you just never know what your players will do...