So, I don't know whether or not this problem is mod related, but I figured I'd give it a shot in this section of the forums first.
I activated the Go Fishing quest, and have collection 12+ Rumar Slaughterfish Scales. However, when I talk to that jerk of a fisher, it doesn't register that I've collected the necessary amount, nor was I ever given a message about completing the quest. What gives? Here's my mod list, just in case that's the culprit:

Oblivion.esm
Francesco's Leveled Creatures-Items Mod.esm
Francesco's Optional New Items Add-On.esm
Oscuro's_Oblivion_Overhaul.esm
Mart's Monster Mod.esm
TamrielTravellers.esm
FCOM_Convergence.esm
Unofficial Oblivion Patch.esp
Unofficial Shivering Isles Patch MOBS.esp
Francesco's Optional Chance of Stronger Bosses.esp
Francesco's Optional Chance of Stronger Enemies.esp
Francesco's Optional Chance of More Enemies.esp
Francesco's Optional Leveled Guards.esp
FCOM_Francescos.esp
FCOM_FrancescosItemsAddOn.esp
Fran Armor Add-on.esp
Fran_Lv30Item_Maltz.esp
aaaWindSpear.esp
Natural_Water_by_Max_Tael.esp
Natural_Weather_HDR_by_Max_Tael.esp
Natural_Habitat_by_Max_Tael.esp
Natural_Vegetation_by_Max_Tael.esp
ExnemRuneskulls.esp
FCOM_ExnemRuneskulls.esp
Knights.esp.bak
Bob's Armory Oblivion.esp
FCOM_BobsArmory.esp
Loth's Blunt Weapons for Npcs.esp
FCOM_LothsBluntWeapons.esp
Oblivion WarCry EV.esp
FCOM_WarCry.esp
Oscuro's_Oblivion_Overhaul.esp
FCOM_Convergence.esp
FCOM_SaferRoads.esp
FCOM_MoreWildernessLife.esp
Mart's Monster Mod - Extra Wounding.esp
Mart's Monster Mod - Foxes.esp
Mart's Monster Mod - Gems & Gem Dust.esp
Mart's Monster Mod - Hunting & Crafting.esp
Mart's Monster Mod - Less Bone Loot.esp
Mart's Monster Mod - Looting NPCs & Creatures.esp
Mart's Monster Mod - Resized Races.esp
Mart's Monster Mod - Zombies for Body Meshes.esp
TamrielTravellers4000.esp
FCOM_TamrielTravelers.esp
T.I.E4MODS.esp
T.I.E4MODSOOO+FCOM.esp
MidasSpells.esp
Knights.esp
DLCShiveringIsles.esp
Mart's Monster Mod - City Defences.esp
FCOM_Knights.esp
FCOM_UnofficialFranArmorAddon.esp
FCOM MWL-TheHeath patch.esp
FCOM MWL-RollingHills patch.esp
Bashed Patch, 0.esp

Did you collect the scales before starting the quest?

some mods you have place others slaughterfishs around... make sure you got the correct scales. follow the green road... ops... green compass marks.

Right, I quickly realized the difference between the types of scales. I'm using the correct Rumar scales. And I did not collect the scales first. Only after receiving the quest.

Seems this quest have indeed some reported glitches and is fixed by the unofficial path, you must consider using it together the ones to SI and other DLCs you have.

If already late to save the issue you can try typing in console:

setstage SQ09 90 <--- this will bring the quest journal entry you have collected the 12 scales

if is not enough so you can 'close' the quest with console:

setstage sq09 100 ... so to remove it from the active quests (you lose the prize but...)

[SPOILER Warning] - You should get a message that you've gotten enough scales, once you've followed enough markers and killed enough of them. Also, I noticed when I talked to him, that he didn't immediately acknowledge I had them. I had to talk to him, and he said something about needing them, and then I chose the "About the scales" option (this is the exact same sequence you'll get before getting any of them); at which point he then realized I had them. It's a little confusing, so if it looked the same and you exited without talking to him, try again.