Net Gain solution
by Kegler

Fourteen answers have various fish added to become new words.

Net Gain solution grid

Across
1CHUBBIkER
7A(SPEC)T
12H(U,L)A
13T + ROOP (POOR rev)
15FO(RT)TUNAE (REF TO anag)
16RU(N)E
17AFTE + R (FATE anag)
18GNEISS ("NICE" hom)
19PI + T(M)AN
21GARDEaN
23SEPO + Y (POSE anag)
24PAR(ALL + EL)'S
26M(I)LK'SHAKE
30AVON (rev)
31SPAN + IS + H
32BASSETS (SET anag)
34A + C + CRA (CAR anag)
35kETCH
36LEAD (2 defs)
37EKED (DEKE rev)
41CARPE + NTER (RENT anag)
43M(A)EN + AD
44SEA COW ("KEY SOW" spoonerism)
45UNRELATED (ELDER AUNT anag)
46CEDE ("SEED" hom)
47SEC + RETE (TREE anag)
48DECOD+ ED (ED rev)
Down
1C + HEAPSKATE
2H(mUFFIn)ER
3BARE ("BEAR" hom)
4BE + TRAY
5kERNEl
6ROAN (NORA anag)
8S(PEN)D
9EQU(I)NE (QUEEN anag)
10CONSOLES (2 defs)
11TH(E + S)IS
13TU(R)NPIKE
14F(REELANCh)E
18G + RASPED
20MOO + NSHINER
22shARKS
25POAC + HER (A COP anag)
26MUS + I + C + ALE (SUM rev)
27L + ITERATE
28K + ICK
29SHADO + WE'D
33S(A + P)PED
34APN(E)IC (PANIC anag)
38ER(O)DE (DEER anag)
39E(M)US (SUE rev)
40CAN + E
42TACO (hidden)